Thatís the light-blocking material coming loose from the baffle frame. It will have to be re-glued or replaced by a competent technician with access to new Mamiya repair parts.
This causes light leak problems and will get worse as more of the material detaches from the baffle frame mounted behind the reflex mirror. When it gets really bad you wonít be able to change a lens without totally exposing the frame in position. Eventually, the camera will become unusable until the baffle assembly is repaired.