Those are the best pics I was able to take before the unidentified creature disappear in a LR opening. It' very small, 1/4 of an inc., and it moves as a slug
Flatworm.
If there's only one, siphon it out.
Francesca, it's ALWAYS hit or miss with any animal that supposedly eats fw's or algea or bacteria (cyano). If you've taken five fw's out already, I'd be willing to bet the ranch you've got 100's you can't see hiding within the rocks. I'd do the Salifert FWE treatment before it gets to plague or even nuisance proportions.
After you've eradicated them, make sure you treat every single new coral addition with a few drops of FWE in the acclimation bag before introducing it to your tank. Just a precautionary measure. You'll be glad you did.
HTH,
Russ
I got 2 more, now I have 5 in a jar waiting for the fwe to arrive to became my live test.
Why is everyone bent on destroying the flatworm? Has anyone positively ID'd this one as destructive yet? Why dose your entire tank with FWE if it's not known whether or not it is absolutely necessary? There are so many flatworms many of which do nothing to harm your tank. One of the greatest things about our reef tanks is that there are so many things in there that we may never stop finding new things in them. Being cautious is one thing but just eradicating specimens without knowing for sure is not the best way to go about it. Also, I haven't seen anyone offer up the sixline wrasse as a possible flatworm eater. In my experience they are quite skilled at it. I've also seen a lot of flatworms in my time in the hobby and have yet to find any damage to any of my corals, in store (displays) and out.