Some people seem to occasionally get a "bad apple" peppermint that goes after polyps and desirable corals, but I think this is the exception rather than the rule. I have kept many pepps and never had them eat corals, right now the ones I have are on a anti aptasia diet though. In the past pepps have always eaten aptaisia for me, I don't know what these shrimp's problems are.
Note that just because a store says something is a peppermint does not make it one, so it would be wise to know just what you are looking for.
I went to the LFS awhile ago to buy some pepps. The employee asked if he could help me.
"I am looking for some peppermint shrimp."
He takes me to a tank with about 30 camel shrimp.
"those are camel shrimp."
"They are peppermints, they are reef safe."
"they are camel shrimp and they are not reef safe and besides, I want peppermints specifically to eat aptaisia."
"Yes, they are peppermints and we sell them to eat aptaisia."
"well, I don't want any of those."
I went in later and grabbed the owner and took him over to the tank and asked him what kind of shrimp they were ( I knew but I wanted to see if he did) and he said they were camels.
LOL, moral of the story, know what you are looking for and don't rely on ill informed employees!