I have these fluffy little things all over the glass and few surfaces in my tank. I think what ever it was hitch hiked on a plant I purchased. I noted them on the glass just after I put new plants in a fishless tank with other plants. I scraped the glass and I didn't see anymore for 1-2 weeks. Tonight I was watching the 6 Lemon Tetras, that I purchased a week ago I have and noted these things were back on the glass. I saw pictures of Hydras and it doesn't look like that. I've seen pictures of snail eggs and they look more like that. But really to me it looks like little dandilion fluff. They attach by a small stem and float in the currents.
I can clean them off with my gravel vac and they held pretty tight.
Any idea what they might be?