Flat worms - Will a Mandarin eat them
 

I just noticed my first flat worm in my tank, at first i though it was another snail trail on the glass, but nope, definitely a worm. Its really thin (about 2mm), but i would guess it to be about a foot long. Are they harmful to the fish or anything else (corals) in the tank? its stuck to the glass in front of my anemone, but easy to remove if i should remove it.

If its a flat worm it will not get big. Yes syphon it out if it is. And for the most part they eat SPS. If you see 1 you more than likely have many more. A pic would help for an ID too. As for the mandarin im not sure

If the worm you are talking about is about a foot long and 2 mm thick it is not a flatworm.

Remove it while you have the chance until you can at least ID it.

Has it moved? Is it squiggly? Might be Cerith snail eggs.