I am kind of worried about it being a hitchhiker bubble tip that is multiplying rapidly.
It would be really really rare for a BTA to hitchhike.
But, it would be very typical for a majano anemone to show up on live rock, and they spread like crazy. I made the mistake of feeding my majanos, because they have bubble tips I thought they were a BTA's, now I am seriously considering throwing out much of my rock, or at least putting it in the backyard for a while to completely dry, because the mojanos have take over.
Majano anemones are pests, they multiply like crazy, have bubble tips, and are rarely more than an inch in diameter.
I really hope you have a true BTA, but thought I would put out a warning to you so you don't end up like me.
Here is a pic,