I had a few like that on some older M200s - I think simply flushing motors with fresh water after use in salt water isnt enough as clearly salt can still get trapped here over time. Another salt trap exists in the M200 mounting frame itself around the metal thread inserts. When these become salt/rust siezed you cannot remove the motor without destroying the mounting frame. I think the solution is to occaisionally dissemble the entire motor, clean and apply some anti corrosion grease like CorrosionX?
EDIT- actually dont do this as petroleum products will damage the plastic parts of the motor housing (see thread).
Similarly some stators can be susceptible to rust an so should be regularly cleaned as well. I’ve also noticed some scratches on the motor spindles presumably caused by the lug screws in the part you show becoming loose. So seems important to check this occaisionally and re-installed with loctight to prevent this from happening as might contribute to the issue you illustrate.
Another related maintenance issue relates to something @tony-white once referred to as ‘heat-cycling’. I think this refers to the repeated stess on plastic components caused by fluctuations in environmental temperature. This can cause the M200 mounts to crack (usually fine hair cracks) around the metal screws as the screws expand in heat and cool when placed in water. Solution would be to avoid leaving the ROV in the sun for any period (as advised) and store in a cool place with a constant temperature (eg. not in a periodically heated room). Also overtightening any screws (as advised) might increase the likelihood this will happen.