Well, I've never seen anyone who's had a tank overrun by those. So it's not a common pest, at least. Then again, I've never seen those at all, so I don't know if it has the potential.
There are an assortment of small types of anemones that come in on rock sometimes, that can split at a notable rate but don't tend to spawn and get everywhere.
A quick test you could do- use tweezers to touch a mysis shrimp piece to its tentacles. If it sticks strongly onto the mysis and quickly curls up to eat it, it's likely to be an anemone. Corals will certainly do the same, but anemones tend to be a bit stickier. You'd also give it some fuel to grow a bit faster, and possibly give a quicker clue about what it is. Even if it is a pest, one or two anemones will take awhile to turn into a big problem, so you have plenty of time to figure it out.
I'd suggest making your own thread, titled something along the lines of "weird probable-anemone", to see if you can get the anemone lovers to come out of the woodwork and help you figure it out.
