Doesn't look like the notorious red planarian "flatworms" to me.
Really, to tell the truth I can't see much from the photo.
Flatworms look more like a mini version of a mermaid's purse (shark egg case), but usually clear to brown or reddish in color.
They don't really do much harm, except multiply so quickly they smother stuff. Usually a good siphoning and low detrius buildup will keep them in check. Salifert's Faltworm exit or Green Ex work well in chemically removing them from your system. Be sure to siphon out all the dead flatworms and do a large water change and run carbon post treatment.
AFA amphipods or copepods, as RL said most are harmless, detritivores, herbivores... Then there are the "red bugs" that are found on the acropora, I strongly disagree with Schmeik after seeing a closeup video of these guys leterally headbutting the acropora tissue and chomping on it.
Anyhow, I don't see red bugs or "flatworms."
Most fish as a biological control are hit or miss. IMO like RL suggest 6 lines seem to be the best, Peppermint shrimp tend to eat a good number of pods also.
I'm a big fan of just pulling out the good old 1/2 inch tubing attached to some rigid tunbing and just sucking out as much as I can. That's what I did with my flatworms until I finally just decided to use the Sailfert product.
But again I doesn't appear to me that you have anything very harmful there.
Mike