Yep that's flatworms. Freshwater dip gets them off rock and corals pretty fast.
Been there, done that with the chemical fix - I had a huge infestation a few years ago and the treatment caused such a die-off of the flatworms that they fouled the water and I lost 7 of 13 fish
The populations of them will wax and wane naturally - I'd FW dip to get rid of most of them - and leave the rest. Unless they're so dense that they're smothering things, I haven't noticed any other ill effects. The "cure" was worse in my case.
Jenn
__________________
Member of the "J" Crowd & the BRW Crowd!
LFS Owner:
Imagine Ocean

Just keep skimming, just keep skimming, just keep skimming, skimming skimming! What do we do? We skim, skim, skim!