That coating definitely does not look normal! It looks like it was scraped off by some debris that likely got caught inside the thruster, between the rotor and stator. Without the coating, the magnets can start corroding quickly, so it is strongly recommended to replace the rotor. We have spare rotors available here.
Hagfish slime sounds like a nightmare to clean out of everywhere! Luckily, we haven’t had a run in with any yet during our dives of the California coast.