It's used as an abbreviation to say “Go Google it yourself” when someone is too lazy to look up information.
The kanji "Kasu" may literally mean scum alone, but the proper meaning is more about feeling than being an actual insult. Is to be read the frustration of a person as if saying “Come on, just make a little bit effort to search by yourself.", because the course of action should have been obvious. And among online people, this is pretty tame and mild compared to actual insults that do use offensive wording.