anemone

[sumpfinfishe]

Looks to me like a Rhodactis-AKA hairy mushroom.
With lot's of food these can mutiply like aptaisa and can reach sizes well over 12" in diameter.
My old roomate use to have two that were both over 8" in diameter.
Not only can they multiply out of control, they can and will eat unwary fish that settle down for the night or swim to close to them.
My roomie finally traded his in after the larger one consumed most of an orchid dotty back, we new this as the next morning it was spitting out the remains.