LOL peed in the radiator... Maybe got a little enthusiastic with the sandwich he was eating over while filling? It's not so weird considering the old egg in the radiator trick.
Agree it looks like stop leak.
If it's not hard crusty bits just flush it out? AF is cheap, flushing easy. I'd be less worried about the cap full of crud and more worried that stuff's built up in your heater core. Still, easy and cheap fix. If you flush your core, do yourself a favor and get the hose clamp tool that has an extension. And rubber hose. And a sprinkler adapter to go to your garden hose. And towels. Lots of towels.