Looks like a limpet, in which case it would not be flat like a flat worm. It would also have a shell (unlike a flat worm.)
How quickly does it move? If you swirl the cup that the animal is in, does it lose its grip w/ the bottom?
You are puzzling me. Photo looks just like a limpet, and nothing like a flatworm. Do you have any Stomatellas? These are snails that can detatch part of their foot when necessary and use it to distract preadators. I have very small hitchhiking limpets that are described as "China men's hats" that look like the animal in the photo. Where did you find the animal? There are small photosynthetic flatworms that often are found on corals that can be a golden-brown color.
Baby Stomatella, cool good guy.
Stomatella tend to come out more during the evening and at night. If you have flatworms you maybe able to find more on the stocks of softcorals and mushrooms.Hmm I can hope but I don't think I'm that lucky. It goes flat when the water is off him thats why I'm thinking flat worm. I sure can't find a pic of anything that looks similar.