malnutriton before getting the peppermint, or while it is in your the tank, can possibly determine their aggressiveness in taking an oppurtunity to get food. i normally feed a lot in my tank, i have never had problems with peppermint shrimp, but i got a starved moray eel from petco and it is so aggressive to get food it goes for my fingers constantly, even though i am feeding it almost every other day which is over the reccomendation.
just a thought from experience....