Damper fluid that has leaked and dried usually looks like a dark greasy mess, not sawdust.
I think a destroyed bumpstop, which could be made of something like cork and disintegrate like that is the better bet.
In any case, you need to pull those coilovers and disassemble to take a better look.
A bumpstop is a replaceable part, but if more things are wrong it's time to replace or rebuild those coilovers.
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