I hope this is helpful. I love silver dollars and have had Metynnis spp. (4 species) for years, and although I also keep other types of fish, these are my favorites. That is why I have taken a personal interest on your fish.
Yours is of the spotted silver dollars, this one
Metynnis lippincotianus, one of the most commonly available in the trade. I believe it is a female, but seeing only from above it is hard to tell. SD's are extremely social fish, living in groups with hierarchical structure, and moving around as shoals; they do not do well as single fish, and being alone is by itself a major cause of stress (even for such hardy and assertive fish!). When under suboptimal conditions, they can be very skittish, which often leads to darting around and injuring themselves against the tank or decorations. Under better conditions, they are not skittish, but are certainly active and fast. They are also long lived animals (decades) and are very resilient so often people feel like "I have had it for years (or months) and it has been just fine, only recently......." In reality, the fish has not been fine, but being very resilient, it has taken a long time for negative conditions to show and decline to become evident. Most M. lippincotianus are bred massively in Asia (less commonly in the USA), brought to the USA (and other countries) and sold as babies. Relatively rarely, wild collected fish are available; if so, usually sold larger, more grown up. They are very hardy and adaptable by prefer relatively soft water, lower or hovering around neutral pH (but can take slightly higher), and will not thrive under poorly oxygenated water. Nitrate levels should be low, ideally undetectable. They like places to hide, but also areas to swim freely. Contrary to popular belief, they are NOT strict vegetarians; in fact, they are omnivores who include lots of plant matter (plants, algae, seeds and fruit), but also animal matter (insects, crustaceans, and 'worms'). They do not do best (but sadly will survive) in strictly plant-based diets, and certainly if the plant material is mainly things like lettuce and spinach.
Information about your tank will be necessary, specifically water parameters and characteristics, such as:
pH (you stated about neutral), hardness, temperature (75-80 is fine), and most importantly, typical levels of nitrate (both before and after water changes). In addition, frequency and volume of water changes (% per week or equivalent), number, sizes, and species composition of tank mates. I doubt other fish are directly harassing the silver dollar, as usually they are very assertive. Also frequency, amounts and composition of food items. All of these are likely to inform how to improve conditions, which is the ultimate cause of the problems shown on the fish. In addition, I believe your tank is 65g; is that 48" long", or 36" long? A picture of the full tank will help envision conditions.
Your fish has two completely separate issues, neither of the two is directly contagious to the other fish in your community tank. Regardless, all fish in the tank have been exposed for a long period of time so there is no good reason to keep the SD in the bucket, as that will only contribute to his decline rather than improvement, and will not protect the rest of the tank from something that is already there.
The pitting around the head and gills - This is common in SD's, and is usually caused by stress over extended periods of time, usually associated with suboptimal (or right down poor) conditions. Those may include, i) suboptimal water quality or water characteristics, ii) limited space, iii) living alone (no other dollars), iv) suboptimal diet, and most likely, v) combinations of all those issues plus potentially others. Although not the same thing, this is somewhat analogous to pitting on the head of cichlids, and in the same manner, lessions can become secondarily infected by any of a number of bacteria and or fungus. The condition is reversible, sometimes with minimum scarring, but it will take a concerted effort to improve as many of the potential sources. That means, water quality needs improvement (via water changes), diet modified, ideally suitable tankmates, best more silver dollars (but I realize that can be difficult).
The whitish stuff on the caudal fin, which may also appear sometimes on other fins and then go away temporarily, only to reappear or become larger - This is also very common, and I do not know the proximate cause, but it is always related to suboptimal water quality. I strongly believe the causal agent is either bacterial or fungal, but I feel more strongly about bacterial. It often reduces itself (of even disappears) after a water change, only to reappear. But it is there, if not on the fish, I suspect hiding in the substratum, only to reinfect later. If not addressed, it can result in permanent damage to the fins (and worse), and become secondarily infected with additional bacteria and fungus. It is also almost always reversible, with little permanent damage, if corrected.
Again, neither of the two conditions are directly contagious to the other fish in your tank. They are present in the tank, even if they have not manifested themselves on the other fish, or you may not have notice them. Some fish deal better with long-term stress or water quality than others.
I apologize for the long text, but hopefully it will be helpful to you. In time, if you want suggestions for better food items, I would be happy to address that. Please put (the dollar) back in the tank.
I leave you with photos of some of my dollars, including some M. lippincotianus. Cheers and good luck!
A 75g with 3 spp of Metynnis (only 2 visible). Three fish in the picture are an estimated 24 years of age (20 directly documented)
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Some of the same fish in a 125g, 3 spp.
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Same tank, 2 years later (last year)
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Metynnis fasciatus (tiger dollars) in 75g
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