It appears to swim sort of like a squid. I put live sand in my sump/refruge last weekend and now I have these as well as a whole bunch of other free swimming creatures that are very small In the green picture you can see what the others look like in the upper right of that photo..
Probably a flat worm. I don't know if it's a "bad guy" or not... my guess is probably not. The other organism is an isopod... harmless and fun to watch.
I also took the opportunity to thin out my macro algae, which I found a small growth of bryopsis and promptly pulled that hunk of rock and put it in the freezer. I also moved a couple turbo snails to the refrugium.
flat worms are bad, but i once saw a handful of these flying carpet looking things that Tropicorium had in there to eat flatworms that were the most amazing things i've ever seen, and would build a tank to feature if i had the money. I think it was a nudibranch.