I hate to say it, because it's beautiful, but I think it's a Majano anemone, though the tentacles are pointed. Majano usually have rounded or bulbous tentacles. Doesn't at all look like an Aiptasia, but that doesn't rule it out. Could be a Zoanthid polyp. Have you noticed it moving at all? If it moves, it's most likely a Majano Anemone. They can move quite some distance in a short period.
I have seen Very brightly colored ones for sale on ebay. Hot reds and greens. They brown out as in lose color, when moved. Some people like them...for awhile
I wouldn't recommend watching it. It'll end up moving quickly, and you won't find it...lol. Get it out. If you can remove the rock, they're much easier to remove than aiptasia. You can usually pop it off the rock with a butter knife or razor blade, but don't cut it. You don't want to leave any pieces of it behind.