I guess I didn't know ammo had all that much air to leak. . .
It could be a bit of rust from the steel case oxidizing. As long as it hasn't began pitting the cartridge it'll shoot safely. Just clean it off and carefully analyze it. It sounds like you don't have too much experience with firearms, so you might wanna do some reading up on how to tell if your ammunition is safe. 7.62 is pretty cheap so if you have questionable ammo just go buy some new stuff. I have found that mil-surp wolf may
be very mildly corrosive even though it's labeled non-corrosive, so it won't hurt to squirt a few drops of Windex (ammonia) down the bore and in the gas block after shooting.