There are a couple different species commonly seen but all are small (about 1/16 of an inch or smaller).  These reddish brown "flat" worms will appear on the tank glass, the sand, on your corals ... on everything in the tank.
    

Flatworms do not eat or hurt corals but if allowed to grow in enough numbers they can block a coral from getting enough light so we want to reduce their numbers as soon as possible as they'll just continue to multiply.  Flatworms have toxins in their flesh that can be harmful if eaten by fish BUT I've found 2 natural predators that aren't bothered by them & actually eat them as though they were a delicacy!

 Scooter Dragonet!  A cool little fish that scoots about our tanks always looking for tasty copepods and more to fill their never ending appetite!  These unique fish are not only fun to watch but have proven themselves to be great at eating flatworms too!

Skunk Cleaner Shrimp!  Another critter that is a fantastic clean-up crew that LOVES picking up & eating flatworms!

Flatworms are usually an indicator of high nutrients in a system.  Though the 2 above livestock remedies are great but typically they need our help to reduce the number of flatworms initially.  The best way for us to knock their numbers back is to gently siphon out all visible (as much as possible) perhaps a couple times a week (small water changes which also helps reduce the nutrient load). A helpful tool I found are Micro Vacuum accessories.

    The kit comes with many parts we don't need.  The brush I've circled is the one I use & works the best.  I use a 1/2" ID siphon hose with a piece of 1/2" ID rigid tubing (for control - cut to whatever length your tank needs for reach) then insert the brush into the end of the rigid tubing.  You may need to wrap the end of the brush with a little bit of duck tape etc. to make a tight fit.  Not hard nor expensive but a wonderful tool for this & other siphon jobs that need a little brush to help lift the material into the siphon hose.  You can purchase this Micro Vacuum kit on Amazon or often at your local Home Depot in the shop vac isle.

Another remedy is to physically remove affected corals & rocks, bring them to your sink & quickly rinse with lukewarm water.  Sometimes I fill a pan with lukewarm water & gently submerse & swish a coral for about 3 seconds then immediately return the coral to the tank.  Flatworms cannot tolerate freshwater & will detach instantly.  This freshwater removal is only recommended for soft & LPS corals.  I would not try this remedy on SPS.

Add fresh ROX reef carbon to your system, perhaps even a Polyfilter & be sure your skimmer is operating properly because again, Flatworms thrive in nutrient rich environments.

Try not to fret over Flatworms.  They aren't hurting anything & when the nutrients are lowered, many times they will disappear overnight.