Once again I have flatworms and for those who have heard about flatworms but have not yet seen them I present to you two pictures the first is my Watermelon mushroom the brownish flecks are flatworms the second picture shows on my hammer a couple specimens of flatworm oval in the front with 2 tails in the back.
These are common flatworms and not harmful per se to coral however in plague proportions they can cover the entire surface of coral and prevent sunlight from reaching the coral and they can starve the coral of needed light causing the coral to not fully open and without treatment can eventually kill the coral from light starvation. The sad part about flatworms is they breed like rabbits and in very short order can reach unmanageable levels in less than a few weeks meaning that they are literally everywhere on rock, sand, coral, glass and piping.
There is chemical means and natural means to remove flatworms. First natural means unless you buy a nudibranch that eats only flatworms you will not eradicate them only control the population. Most wrasses eat flatworms (Yellow, 6 line, Leopard, Melanrus, Dragonets to name a few) however these can be hit and miss as they may not even eat the worms or just nibble at them. Other means of controlling flatworm populations are most species of shrimp and certain species of crab and even some species of snail will munch on them however just like fish they can be hit and miss and unfortunately you can buy all the above and populate your tank with it but have none of the fish or critters touch the flatworms.
Chemical means there are many chemical means out there but the most effective is Salifert Flatworm exit it is reef safe (I have used it several times) and will not harm your corals but may make them unhappy for a few days, fish or inverts and if you follow the simple directions you will not notice any effect on your tank at all. However with that being said when flatworms die they can release toxins in the tank a few dozen releasing toxins most likely won't hurt the tank but a few hundred releasing toxins can definitely hurt the tank and a few thousand releasing toxins can initiate a tank collapse so it is very important that as they die try to get out as many as you can.
With all that being said no matter what chemical you use remember to read and follow the instructions.