Disc brakes* on motorcycles are very exposed. They pick up a lot of dust and crap and get contaminated.
I have, and have had several bikes that liked to get sqeaky all the time. Sometimes, just applying the brake really hard will get rid of it, for a while.
For just mopeds, I’d consider roughing up the rotor and pad with something and cleaning it with brake cleaner. What type of disc brakes are these BTW?