It is normal for any fish to pick on the smallest, but not OK. Sounds like you need to remove him. Just because the mbuna isn't trying to eat him does not mean they will not try to damage him and accidentally kill him.
Do I recall that you have 5 polit and 5 demasoni and 5 of something else?
Majority of my fish are over two inches while the tiniest one is 1.25". I have 5 of each species. I need to buy a quarantine tank but I have to cycle it first which takes weeks. Just by observing them I cannot tell which fish is picking on him yet but maybe it will become apparent sooon.