That looks like the C clip off the end of the baffle tube, meant to stop the mirror from falling forward off the baffle if the focuser is disconnected. If so it's easy to fit back into the groove, then perhaps do a minimal clean of the spots on the mirror without removing it.
It's not hard to remove the mirror, but kind of a PITA dealing with the grease if you don't really need to.
Remember to keep the same rotational orientation of the corrector plate during reassembly.
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