Okay, so this is an old thread but that "polyp" or whatever it is has multiplied and switched the direction that it was sitting.
It only comes out at night or in the dark. It's also multiplied...though you can't see it well here, there are three of them now all looking in different directions. If you look down and left beside the larger mushroom, you can see one of the babies though it's color is muted.
Here is the pic:
Coloration is a brownish with fairly bright green blotches in the center. It is almost an inch across now.
Again, never had aptaisia in this tank...haven't introduced anything in a long time.
It could just be a polyp but I've not heard of many that only come out in the dark and close all day (even under the overhang that protects it from light). I have had a few small fishes disappear and it seems that when they do, this doesn't come out for a few days around that time. Otherwise it's out every night.
Any thoughts?