The vacation feeding block might have dissolved quicker than it was meant to and released all the food at once. This could have caused an ammonia or nitrite spike that poisoned the fish.
Big daily water changes and gravel cleaning for a week usually helps the fish, however permanent damage might have been done.
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The dark green slimy stuff on the rock is blue green algae (Cyanobacter bacteria), which is a form of photosynthetic bacteria (bacteria that can use light like plants do). It loves nutrients and if it started growing while you were away, the vacation food dissolving quickly might have contributed to it's growth. However, changing filter media regularly and lack of water changes and gravel cleans can also cause it to grow.
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Filter media/ materials should only be changed if they start to break down. If you replace the media, you get rid of the beneficial bacteria that helps keep the water clean and free of ammonia and nitrite.
Filters should be cleaned once a month. The filter pads/ cartridges/ sponges can be squeezed out in a bucket of aquarium water, and the media re-used in the filter. The bucket of dirty water can be poured on the lawn.
If you have filter pads/ cartridges, you can replace them one at a time, with a piece of sponge for a different brand of filter. Just find a filter sponge that is the same size or slightly bigger than you need, and use a pair of scissors to cut it to fit in your filter. Sponges last for years and get cleaned as described above.
