Gravel and mold issue

[GayleH]

So last night I discovered what I think is mold in the tank, it looked like a dandelion exploded. Today I used the gravel vacuum to get rid of some of it but there is still quite a bit. I knew when I got the tank that it needed a good cleaning but this is getting out of hand. So my questions are, is my filter strong enough to clean the water? It's an aqua clear 20, brand new but doesn't seem to help, and is my gravel too small? It's really tiny, and I can't seem to clean it well as it is covered in algae. I want to replace the gravel with something bigger but don't know if that will help. I haven't added any plants yet as I want a healthy tank before hand.

I've had fish most of my life but these are my first warm ones and never seen the dandelion issue before.
20 gallon, 1 plecostomus, 3 neon tetras, 3 black neon tetras, one frog, 2 male platys (which is probably another problem).