Hydra. Not a bad thing, just ugly, believe me I've seen way worse "things" in some if my tanks before such as transparent mushroom like things that almost killed one of my fish.
hydras, I think, are awesome haha. I have them in a 55 with my pickerel and sunfish. There will be occasions that I may feed one of the larger ones with a blood worm or two haha, interesting to watch for sure.