I'm pretty sure this is NOT aiptasia, b/c of color, the way it closed up when I touched it, and the fact that the adjacent Anthelia sp. does not appear impressed by it at all. That said, those tentacles scare me, so I wanted to see what y'all thought. Thanks!
I'm the kind of person who would wait and see. It is definitely not in the usual Aiptasia family. Doesn't look like your stock majano either. There are many other species of anemones and even the giant anemones start out really small.
Now if I saw 2 or 3 show up over the next couple of weeks while I made a more concerted effort to ID this, I would probably have to start thinking about killing them..
but that's just the "curiosity for seeing something new and cool and learning about what it really is before I freak out" in me I guess.