Unless the store charged you extra for it, I wouldn't be so mad at them. 9 out of 10 times, the algea they sell came from another hobbiest as a trade-in for credit. I worked at a LFS for 3 years, I saw my share.
While we did our best to screen it, those little buggers can manage to get in there..
And yes, its definately an aiptasia, smart move removing it.