Normal playfield generated stuff. Usually from bumper, slingshot or flipper mechanism components. Moving parts wear out.

That dust looks like brass filings.

If it were mine, I would be concerned and locate the source of those particles.

Plumb bob perhaps?

investigate. not normal.

I know exactly what that is! Had the same on my tron. The shavings are brass from the coil stop on your left flipper. I wouldn't worry about it too much, just get a replacement on order the next time you order for when the left flipper starts to stick up. Find my post in this thread to see a picture of what's wearing:

http://pinside.com/pinball/forum/topic/tron-upper-flipper-sticking

My Tron did this also. Also only on the left flipper. New coil stop should take care of it.

My Star Trek pro is doing it on the right flipper. My south park did the same when I rebuilt the flippers. Totally normal in Stern pins.

That washer is a spring washer spacer. It supposed to be bent like that, to take out any play in the coil. It basically helps keep it all tight so the coil doesnt shift/move between the mounting brackets when energized. All normal there.

Holy crap that coil stop/sleeve looks like it's had years of play!

Erak is correct. I would take out that washer you put in and put the spring washer back in.

I am having the same issue. I have cleaned the shavings out several times and they seem to come from all the flipper coils. My cabinet get covered with shavings on a regulars basis so I guess I'm in for a rebuild on the flippers soon.

Seems like a serious lack of quality control...

Wow.. after reading this thread, I checked my STLE too and there's the same brass glitter coming from all 3 flippers. after just one month of home use, the coil stops look worn-out like years of heavy usage, compared to the other Stern machine in my game room. is there something else that can be adjusted to prevent this heavy wear?

I'm not sure if this will help, but you might try moving the 'wave-washer' from the stop end to the bracket end of assembly. That way the coil is already pushed back to the stop as far as it can be. As erak said, that helps eliminate coil movement in the assembly, which contributes to the 'buzz' effect you get on a worn flip assembly in the hold position.