You can try a couple of sapphire damsels, which actually play nice in most aquariums. I've read those nudis will also eat flatworms but never felt it was worth the risk. I agree with Jose.
I actually had a velvet nudi a few years back when my 50 gal cad lights got flatworms. Slow acclimation was a winner for me. One of the most unique and beautiful creatures I’ve ever kept. When he finished all the work I passed him along to a fellow reefer.