Alright guys, I need your help on this one. Not quite sure this picture 100% fits this site, but you're a smart bunch so I figured I'd ask your opinion.

What in the name of Odin is going on here? What liquid could that be? Is it helping the cup stick to the top or is that something to do with the microwave? I've been staring at this for half an hour and I'm still baffled. Let me know in the comments!

Should have used Flex Seal.