For just a few aptiasia, a lot of things work. Kalk mixed in a tiny amount of water to make a paste works great. Pull it up in a syringe, turn of your powerheads, and inject the paste all over the little bugger. If the rock is easy to take out, I like to burn them off with a small butane lighter. (the torch kind) If you ever get a big outbreak, the Berghia Nudibranch's are the only thing that has ever worked for me. I put 6 in a 30 gal tank and they completely wiped it clean of aptiasia. Peppermint shrimp have NEVER eaten an aptiasia in any of my tanks. They work for some people, but not for me and a lot of others.
The lighter trick is fun. My 13 year old son loves it!!!