Red Flat Worms
 

Are these harmful to reefs? What will eat these, i have a 6line but i havent seen a big drop in numbers yet.

If they are the planaria I think you're talking about, their numbers will drop on their own. Wrasses should eat them though.

So they are not harmful to the reef.

if they start to smother corals, they can be, but mine cleared out before that became a problem. You can turkey baste them off in the mean time.

They are unsightly.