A slimeball is someone who mucus
Mucus is actually exaggerating compliments. If someone is called a slimeball, then that person is exaggerating compliments to giving often. People who want to have something done by others (or who want to make a promotion, for example) can very much mucus and thus be called a slimeball. Often, terms such as slijmerd, Heel licker and Vleier are also called. Generally speaking, the word has a negative tone.