It looks like a clear jelly, sorry the pictures aren't great.
When bought from the lfs (recently) it stated for aquarium use, I boiled and scrubbed it before putting into my 30 gal which is just beginning to cycle with 2 male guppies and live plants, after a week it grew this stuff so I took it out, boiled it again and scrubbed and put back in but its grown back again. What is it? Is it okay to leave it growing? It doesn't look very nice.