Just water from what I've read of other people doing. There is some kind of residue left on certain disks after they exit the factory. I've read that Warner releases are some of the worst offenders and sure enough, I watched Ready Player One last night and it was the first UHD Blu-ray that did anything weird during playback (momentarily froze, then skipped forward one time). Looks like I'll have to try this cleaning thing myself.
TCL R615 | Sony UBP-X700 | Integra DTR-7.8 | Front = Klipsch KLF-20 | Center & Surrounds = Klipsch KG 5.5 | Subs = 3x 15 inch, 2x 12 inch, and 4x 10 inch with miniDSP handling delays and PEQs. Hybrid JBL 2384/2432H/4560/Table Tuba, active, multi-amp trio under construction and testing.