Glock does not contain brass. There is a copper colored lubricant used on new manufacture Glock guns that can be mistaken for tiny brass shavings on the slide guide rail. It's a copper based locktite anti- seize lubricant that you should not be concerned with and don't need to remove. Lube only as required and have fun.

I got a glock 17 6 months ago and noticed it too. I haven't even shot it, just lot's of dry practice. I think it's from the slide guide rails. I put some lube on it. I'll let you know if this is still occuring when I get my ATT and hit the range. Good luck.