Looks like a flatworm of some sort, picture is too far away to positively identify them. However, with as many as you have it's probably a good idea to work on removing them. They can get up to plague like populations.
A natural remedy for them would be a wrasse, which if that doesn't keep the population in check you can use chemical treatment like Salifert Flatworm Exit.
In the future, be sure to dip any coral additions as that can help prevent unwanted hitchhikers.