What is the white residue in a spray pattern seen on the inside of each nozzle of each side core of this Falcon Heavy image from the Teslarati article SpaceX reveals Falcon Heavy Block 5 in first official photo, timelapse. See also SpaceX tweet.
I'm thinking it could be related to engine shut-down, but it looks like they are assembling a Falcon Heavy for launch, so wouldn't the nozzles have at least been cleaned after the previous launch?