Yes they can in numbers and these are Planaria and others are acoel that are less of concern unless in high numbers. The planaria are photosynthetic, so they are often found on the glass on on light-facing surfaces. When bringing new coral specimens home, it is wise to quarantine them as you would a new fish. While in quarantine, the new specimens can be closely inspected for flatworm infestation and can also be easily treated to eliminate whatever flatworms are present before introduction to your display tank.
Flatworms have a number of natural predators, including the Sixline Wrasse, the Yellow coris Wrasse, lunare and melanurus wrasse and the Spotted Mandarin. The biggest drawback to ridding your tank of flatworms with this method is that the fish will not consume every flatworm in the tank.
You can siphon with a 3/8" tube as you see them. If you elect to use a chemical treatment, be sure to siphon all of the dead flatworms out of the tank, as they may contain toxins that can be released back into the tank water as the flatworms decay.