kinda looks like a ball anemone but hard to tell from the pic , if it is explains why its only out at night.google ball anemone and tell me what you think, they are very sensitive the first sign of movement they hide back in the cave:):)

For some reason I just don't think it's a ball anenome. If it is...it doesn't look like any pictures I've seen. It's more like a really good sized Polyp but its more concave than convex which is what many polyps are. The arms also seem to move independently of each other which I have not seen on polyps. None of the arms are that long.

It hasn't been an issue that I can pinpoint to giving me issues but I always like to know what is in my tank.