I have these spots on these 2 shrooms, I have not seen them on any of the 30 or so other ones in the tank. I just bought this rock from the LFS about 2 weeks ago and have seen since then.
Well I compared the photo to what it looks like now and it is moving, I looked upclose and did see one "pull" up an back down onto the shroom, I assume flat worms, I guess next question, what do I do now?
Siphon them out. I had a flatworm problem about 9 months ago from horrible live rock from a LFS I will not buy from again. I siphoned out a lot and bought a sixline wrasse to take care of what I could not reach (bad idea). My flatworm problem was much worse than yours though, from what it looks like you should be able to siphon and be fine.