My daughter's '92 240 has a bad odo, and I'm waiting on the gears for it. Noticed a lot of white powder looking substance on the gauge faces and inside of the cluster assembly. Most of it brushed off the speedo face, the rest doesn't. Any ideas on what it is? I've tried windex and mineral spirits with no luck. I don't want to damage it, so going slow here. Next I guess is to try vinegar in case it's a calcium type buildup, which is what it reminds me of.

I have a similar issue on my cluster. I have heard countless theories about mold, over-spray of some kind, etc. But never really figured out what the source of the dots was. I have yet to successfully clean them off myself, but have heard that regular plastic cleaner can help remove them. The vinegar could be a good thing to try as well. Let me know if that works!

maybe its a dried out protective coating ?

Not sure, we tried vinegar and qtips and it only helped a bit - not sure using water and qtips wouldn't have done the same.
Good news is we got the new gear in and got it all back together and it's working again. I was even able to fashion a new trip reset button from some brass rod and epoxy putty.
Next up is the alternator bushings!