What in the hell is this??

I use this mug all the time. It’s similar to the Yedi style tumblers and all the others alike. Ive never seen this tho and now it’s happening every time. The first time it happened i dumped it out and washed the mug vigorously. Only for it to happen again. I checked the creamer and it’s good until late march. Can someone explain what this is and if it’s safe??