one of the problems with peppermint shrimp is that they wont rely on aiptasia for a food source if they don't have to .. and they normally work best in large groups of about ten .. another easier way to get rid of them is to put the infested rock in a dark tank.. and the aiptasia will come off.. aiptasia diaphana have a very high propogation rate and have no problem thriving in poor water quality. the biggest problems with these little suckers are that since they reproduce so rapidly they can actually become a feeding competitor in the tank and not allow other inverts like your coral to get all the food that they may need.. there are a lot of ways to get rid of them though some easier then others.. shrimp, butterflies, chemicals, injections, bergia, what have you .. while i prefer natural ways others like the chems.. which is ok .. but an injection of boiling water works just as well. and it's a whole lot cheaper.. anyway.. good luck..
Brian