Yes, and I am betting they are photosynthetic. If they are not in the display I would not be too concerned. With this type of fw sometimes a black out can cut back the population if it becomes an issue.
Acoel, brown or rust brown flat worms. likely C. retrogemma
yea its good to keep the population down you probably have more. . they eat detritus. actually beneficial in a way. In large numbers they become a problem.
While Flatworms, also known as Planaria, can become a problem in just about any saltwater aquarium; the aquaculture industry has encountered two distinct types of flatworms that are now fairly common in the United States.
Rust Brown Flatworm
The first and most common type of flatworm can become a nuisance quickly in the home aquarium. These organisms are tan, brown or rust colored with a red dot, and reach a size of up to 1/4" in length. They are oval and slightly elongated with two tail-like appendages at their posterior. This brown flatworm has been identified as Convolutriloba retrogemma, and will proliferate rapidly in aquariums with elevated nutrient levels. The populations of these pests can increase to a point where they will actually perch on corals, and block the light from reaching the corals' tissue.