I'll try for some better pics, but for now I'm still stumped. I had aptasia, and this isn't the same thing. It does seem dull for a sea apple, and I don't think that it is a sea cucumber. Maybe it is some sort of anemone, but I can't ID it.
I have it out of my tank now, but it must have been in there for over a year. I haven't added anything but fish/ inverts to this tank since I started it last January.