Last night I added a blichman brew mometer to my 10G brew kettle. I did a test run to check for leaks and calibration. After I was done I left the water in the kettle with the lid in overnight and today when I went to start a brew day, there is this weird scum, or I don't know what really, with just plain tap water left over night in it... Here's a pic. Any ideas? All safe still? I scrubbed it with soap and water, and it really didn't come off. Is it just cosmetic?