uh oh, is this Aiptasia?

Yup, sure looks like it to me

yikes! kill it quick! inject lemon juice, kelkwasser paster, etc.

They look small enough that peppermint shrimp may eat them.

I had one, when I first started the hobby, and I thought I was lucky enough to have a cool anemone.......... This booger was huge, not to mention the fact that I fed it, and cared for it....lol. Anyway things base was probably 2 inch diameter, and about 5 inches tall. The oral disk was about the size of a nickel, and the tentacles were probably about 3 inches long. Well still thinking I had something, and not knowing that pepermint shrimp ate aptasia, I bought 3 pepermint shrimp. About the second or third night of me having the shrimp, my anemone I cared for so hard disapeared. I figured out a day or 2 later that pepermint shrimp eat aptasia, and aptaisia is a neusance.

Anyway, point being, I do not think it matters what size the aptasia is, with enough shrimp, they will eat it, or I am very lucky that they ate this big one before it decided to reproduce.

Aiptasia can be killed with kalkwasser, or peppermint shrimp eat them. With the number in the picture, I'd say you did the right thing. If the store won't be getting more rock in to choose from, you may want to consider ordering rock online.

Also remember Aiptasia are like fres water snails, once you get them you can never get rid of them, which is why its important to have controls for them.