What is this 'skin' like thing at top of brew pot during boil?

[jigidyjim]

Doing my first all-grain today. There's a weird skin forming on the top if I don't stir it for awhile. It's sort of like if you boil milk, the top layer can solidify or something.

Is that normal? What is it?

 

[DavidHawman]

Probably hot break. It will break apart as you continue to boil, eventually settling out on the bottom of the pot when you cool it.