Flux flusning
I can be nasty looking stuff, can't it? It's also difficult to judge the quality of a joint with the flux present. Commercial boards are cleaned of flux and there's no reason why hobbyists shouldn't, either.
I use either lacquer thinner or acetone to do the job, but beware that you don't want to inhale too much of that stuff. Alcohol is slower, though safer. Most solvents that will remove common flux are flammable.
Certain specialized boards require that you use alcohol to remove flux or you'll destroy the board substrate. Polysulfone boards, used in Tektronix oscilloscope attenuators in their production items of the late 1960's through the 1980's.
Dean