Laser mirror degradation?

What is happening here?

It's like there was a film that should have been removed prior to use, but there was nothing like that even remotely evident on them, previously.

Sticking a Post-It note on there is enough to remove this stuff in flakes.

The mirror in the single photo is the same one that is not mounted on anything in the second photo with a bit more of 'that', whatever it is, removed.

I've bought new mirrors already and am in the process of changing all of it, mounts, head, beam combiner, etc.

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