Yes, they are bad in the thousands, but individually or in small numbers, just a problem to be investigated. What I mean is, don't over react and go chasing all sorts of remedies that may cause more harm if it is not an outbreak or epidemic (I had piles of them all over the tank about ten years ago--that was an epidemic). I would siphon all I see and look for likely causes for them. If you see them increasing or multiplying, then look into remedies or treatments.
Along with the mentioned wrasses a Springer's Damsel sometimes eats flatworms.