But I have never had any idea what they are. My best guess is a deep water anemone. If you let full light hit them they wont extend, put them in low light and they fully extend to about the size of a dime. Their base is spongelike, and they can fully draw back into it so they look like sponges. They have clear tentacles with white tips and a mouth in the middle. I know they arent a problem, because I've had them for years, just have no idea what they are.
Any takers?