A slimy slug is fun, but a slimy person is tricky. Slimy means either sticky and wet, or dishonest and rude. A creep who sells broken wheelchairs while smiling is slimy.
Slime is a moist and clammy substance, like the stuff a snail leaves behind or the mucus that comes out of your nose. Anything that's slimy is like slime, so it's a good word for describing gross substances. If your oatmeal is slimy, it's less than appetizing, and if a pond is slimy, you probably won't want to swim in it. Figuratively, slimy also means "evil" or "despicable."