I build a copper immersion coil recently with grade L copper from the home depot clearance. Last night I tested it by putting it in my new stainless steel pot with boiling water, heat on. I left it there for 15 minutes as if sanitizing in wort then turned off the heat and ran cold tap water through. After I tested it I noticed that this green or blue powder like substance had appeared on the coil in a few places. The majority of it is outside of where there was water contact, but a couple spots on the bottom of the coil exist too. It kind of smudges when I rub it with my finger but doesn't really come off. Bits off the powder like substance do come off on my finger though. I'm not sure what this is and can't seem to find any info on google. I've attached a couple pictures. Does anyone have any experience with this? What is it, is it safe, is it easy to clean? Any info would be appreciated. Thanks.