Flatworm. There are a couple types that are found in reef tanks.
Do you ever see these on your soft corals? I have the "on the glass" ones and the "hang out on soft corals" ones, and I always wondered if they were the same.
I put them somewhere on the spectrum between harmless and annoyance.
There is one kind of flatworm, very reddish bown, and darker, that can form big sheets on the sand and rock. Those can be a real headache as they multiply like crazy and while not overtly harmfull, they are very very ugly. I do not think that this is one of those, but it is hard to tell with the blurry photo.