He probably should tell Childermass about how the Mirror changed him...hell, might as well do it now, with this Childermass who's less likely to give him shit or get into a fight with than the full Childermass.

"The Silver Mirror That It Doesn't Really Matter If I Touch Or Not Because It's Already Changed Me." There's a small little pause before Strange continues. "It's changed my magic as well as my physical being. I've got mirrors on the inside now."

He has no idea how to describe it except that.