Hi,
It actually looks like neither of the options you have suggested to me, but I really can't give you a name for it. The tentacle shape and coloration is really not a match for either Aiptasia or Bartholomea.
Technically, anemone identification is done by removing the animal and examining internal and cellular structure. This is then matched with an external an
atomy. The problem lies in the fact that there are a lot of species of small, tan to brown, anemones; probably dozens of species in the Caribbean, more in the Indo-Pacific, and their taxonomy is poorly known.
We tend to focus on those that well known, such as Aiptaisia or the curly cue anemones, and the others either get ignored or mis-identified as one of the above.
So... I can't get there from here on that anemone; my suspicion is that it is probably harmless, but I also wouldn't place any bets on that.