You are using Hoppe's 9, right ?
The Hoppe's 9 is dissolving a slight amount of the copper fouling. It is then carried to the openings of the mini-y. From there, it dries into a blue-green tarnish.
Mine did the same thing.
To clean it, wet a q-tip with Sweet's copper solvent, and wipe the tarnish off. Follow-up with several more q-tips wet with water to rinse the Sweet's. Wipe dry.
Next time use BreakFree instead of Hoppe's. BreakFree does not dissolve copper and will not cause this problem.