I dont think this is anything to worry about. Unless you see the tentacles not coming out or just generally looks irritated, then you might consider dipping, but if not, just leave it be.
What makes you say they are definitely not flatworms? I've seen clear/white-ish flatworms on fully expanded frogspawns. Looks like they aren't moving but they slowly are.
Try to get a closer look and see if they look like little teeth: