Form1+ purchased new and first print was on 12/23/14. Printing very nicely. Still learning but the machine is performing as advertised.
I use Dust Off to blow off the mirror after each tank change. It may have always been there and I just noticed, or it might have just started. Can you identify this oily yuck? 
This is a view looking straight down at the mirror surface.
Thanks
-c-
At a guess, it’s oil weeping off the Z-stage screw.
I’ve had my Form1+ for a few months now. Dozens of prints, across 4 different resins (with tank swapping) and other than once or twice blowing on the mirror between tank swaps, I haven’t dusted at all. And the printer is still working beautifully.
I don’t think the printer is as sensitive to dust as FormLabs responses would lead you to believe. “Clean the Mirrors” is step one for problem troubleshooting on any optical system, so that’s always the first thing they suggest. But for the people having problems, cleaning their mirrors doesn’t seem to help much if at all. The evidence seems strong that there’s an issue with the laser itself and if you’ve got that problem then maybe the printer is more sensitive to dirty optics. But at least as far as my experience has been, there’s no need to be obsessive with the mirrors if you’re not having any other issues.
I agree with Randy. Its probably lube from the Z axis screw. If it hasn’t gotten onto any mirrors yet its not likely to.
About blowing dust off of the mirror… I know that’s what Formlabs recommends, however, that really just stirs the dust up and creates a MUCH greater chance of the dust sticking to one of the rear mirrors which require a lot more time to clean. I suggest using clean gloves, PEC pads and a little bit of alcohol to clean the big mirror. Don’t use anything other than PEC pads and very light pressure because the Form1 uses “one sided mirrors” which means the metal surface is on the front of the glass and its much easier to scratch than a regular glass front mirror.
Do so at your own risk though since doing anything outside of the Formlabs official sequence of events may void your warranty. You can find forums that explain proper technique in camera forums where they do the same thing to clean the image sensors inside their cameras.
I’ve had to clean the rear mirrors and galvo mirrors because stirring up dust with air blew the dust to the back and got the rear mirrors dirty. My prints were unusable until I cleaned a bit piece of dust off of the center of one of the galvo mirrors. I’d rather not stir dust up though so I clean the mirror vs. blowing it off.
The Z axis lead screw seems to be lubed with silicone/teflon type lubricant.
Those have ridiculously low surface tension and will wick pretty much anywhere you let them (in this case the case, i guess).
It shouldn’t be anything to worry about, i.e. it won’t spray or crawl onto the mirror of its own accord 
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