From what I can see in the picture, to me, It looks like flux residue left over from letting the Isopropal dry on the board.
I would try the isopropyl again only this time mix it with some distilled water (maybe a 50-50 mix). When you are donew scrubbing then wipe it all up with a lint free cloth or blow it off the board with an air hose and nozzle.
Good Luck,
AmigaEd