What is this on my fretboard?

Yeah looks like polishing compound. Naptha and something fine enough to get in there. Maybe a q-tip or a tooth brush.

Or don't worry about it. No biggie.
 
Canned air, like a keyboard de-duster. Solvents may get some of the superficial stuff off, but likely to make some of it penetrate further into the open grain.
 
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