If its a moss powder they should of used it on the other slope. I have never seen a roof covered like this. The whole surface is covered with something. You can see the shingles underneath.
I should of took the picture of the roll asphalt roof that is at the end of the gable roof. It is covered with the same coating.
There was a bucket in the attic and I think they covered the roof trying to seal it and went a little crazy.
I reported that a roofer needs to evaluate before closing and there where other issues as almost every flashing was puilling away and needed sealing. But not with that stuff.
I can see the white residue as a moss powder. I am not to sure about the under layment theory.
Didnt look like a professional roof repair to me.