What is it?

[RRaider]

Some kind of flatworm?

 

[Jimbo327]

Does it move?

If it is flatworm, it should move. Then look for a red dot in the body/head of the flatworm. If you see that, then it is a red flatworm (planaria)

 

[Thhitman]

Those do appear to be flatworms, I have had them twice. Sailfert's Flatworm Exit worked for me.

 

[DetectiveTofu]

Yes sir, looks like Planaria sp. flatworms to me. I'd do some research on them, and get rid of the as soon as possible. When their numbers reach a certain point, it becomes a very large pain to get rid of them.

 

[Azurel]

I had them for awhile and got a Sixline wrasse and he took care of all of them in about a month or so.....