It appears recoating is the only recourse. This secondary convex mirror
coating is damaged with
what appears to be mold that has embedded into it. It doesn't wipe clean.
It stays. It SUX !

If you remove the secondary, which will require removing the corrector, keep
in mind that all of these are mounted in a specific rotation relative to the
primary. There may be alignment marks on the corrector's cell and, maybe,
the secondary's mount, too. The rotation is unique to each scope, adjusted
at the time of original assembly. To get good images on reassembly, you'll
need to respect the correct rotation of the corrector and secondary.
--
Curtis Croulet
Temecula, California
33°27'59"N, 117°05'53"W