Hi and welcome to the forum
The fish does not have Lymphocystis.
It's possible the fish has Tuberculosis (TB), however there is no real way of telling just by looking at the outside of the fish, and there's nothing you can do for TB infections anyway. I don't think it's TB.
It looks like a bacterial infection and is probably caused by the Iridovirus. The Iridovirus is a virus that affects labyrinth fishes (Bettas & gouramis), and weakens their immune system and allows secondary infections to affect and usually kill the fish.
There is no cure for the virus and the best thing you can do is try to improve the fish's immune system by doing big (75%) daily water changes and gravel cleaning the tank. Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
If the filter is more than a couple of months old, clean it if it hasn't been done in the last 2 weeks.
Keep the temperature around 26-27C (79-81F).
Feed the fish a varied diet. If you can get purple and orange coloured foods (purple and orange carrots, blueberries, apricots, orange squash), you can squeeze the juice out of them and soak fish food in it before feeding the fish. The purple and orange foods have various chemicals in them to help boost the immune system.
You can try adding a fish vitamin supplement to the food too. Try to use a powder supplement rather than a liquid one. If you can't find a fish vitamin supplement use a reptile or bird vitamin supplement. You can use the fish vitamin supplement every day. If you use the bird or reptile vitamin supplement, use it 6 days a week and go without on the 7th day.
You might need to treat the fish for secondary infections, like the bacterial infection near the tail. This is best done in a quarantine/ hospital tank so you don't kill the beneficial filter bacteria in the main display tank.
If you can boost the fish's immune system and stop it dying from secondary infections, the fish might survive. However, if it is the Iridovirus, it has a pretty high kill rate (99%).
Dwarf gouramis (Trichogaster lalius) and all their colour varieties are commonly infected with this issue, and the Iridovirus can remain in an aquarium after the gourami dies so do not put any more gouramis or Bettas in that tank unless you disinfect everything and start again with new fish.
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Fish do a stringy white poop for several reasons.
1) internal bacterial infection causes the fish to stop eating, swell up like a balloon, breath heavily at surface or near a filter outlet, do stringy white poop, and die within 24-48 hours of showing these symptoms. This cannot normally be cured because massive internal organ failure has already occurred.
2) internal protozoan infection cause the fish to lose weight rapidly (over a week or two), fish continues to eat and swim around but not as much as normal, does stringy white poop. If not treated the fish dies a week or so after these symptoms appear. Metronidazole normally works well for this.
There is a medication (API General Cure) that contains Praziquantel and Metronidazole.
3) intestinal worms like tapeworm and threadworms cause the fish to lose weight, continue eating and swimming normally, do a stringy white poop. Fish can do this for months and not be too badly affected. In some cases, fish with bad worm infestation will actually gain weight and get fat and look like a pregnant guppy. This is due to the huge number of worms inside the fish.
You can use Praziquantel to treat tapeworm and gill flukes. And Levamisole to treat thread/ round worms.
You treat the fish once a week for 3-4 weeks. The first treatment will kill any worms in the fish. The second and third treatments kill any baby worms that hatch from eggs inside the fish's digestive tract. You do a 75% water change and complete gravel clean 24-48 hours after treatment. Do at least 2 big water changes between treatments to remove any medication.
Treat every fish tank in the house at the same time.
Do not use the 2 medications together. If you want to treat both medications in a short space of time, treat them with Praziquantel one day. Do a 75% water change and gravel clean the next 2 days. Treat with Levamisole after the second water change. Then do water changes for the next couple of days before starting the second round of treatments.
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I doubt the stringy white poop and infection near the tail are related. I would say the fish probably has an internal protozoan infection or intestinal worms and the Iridovirus. Just feed the fish well, do lots of water changes and if the fish stops eating or gets more sores on its body, then euthanize it..
