Fishless cycling a 20 gallon tank?

Um yeah don't listen to the first answerer. 76 is perfect for a community fish tank, and don't restart the tank or you'll have to cycle all over again.

It may be a fungus or a clump of bacteria (although I believe it's fungus). Just remove it (and the gravel that it's attached to) and dispose it.

I would try to remove as much of the white flowery thing as possible. Remove half the water and replace it with tap water and no tank conditioner. The small amount of chlorine in the tap water could help but will not harm the plants. Also make sure you are filtering with carbon. There was probably something on the plants. Before you ever put plants from a pet store in your tank let them sit in an isolated bucket of water for a few days before adding it to your tank. If anything is going to happen most likely it will happen then. Also if you tank light is on for more than 8 hours drop it to 8 or if its close to a window more it. Sunlight can cause some major problems.

76 degrees is way to warm for a fish tank. It will def grow bad bacteria and algea.

I would make the temp at maybe 73. do a water change if not the whole tank and scrub the sides and everything down and start over if you have lots of algea and crap in there.