I've got one too. My tank's only been set up a couple months (since February) and he's just barely above the substrate and in an overhang. It didn't seem to do much, so I didn't worry about it, but now I have 2 - that I can see. I'm probably going to get rid of it today when I do my water change.
Could be a lone zooanthid or mushroom anemone. I have a LOT of aiptasia's, and none of them are purple at all. Maybe the best way to ID is to wait and see if it reproduces as quickly as aiptasia's do. If you can get rid of one, you can get rid of both, so you're not taking a huge risk. Although I've been unable to get rid of any! :evil: My peppermint shrimps are just playing with them.
Sara