SpiderSlime is the pairing of the monster, Slime, and the princess, Penelope Spiderbite.
In "Monster Bash", Slime offers to cure Penelope's spider-bite, which he does. Penelope is very happy, and calls him amazing. Slime leans against the wall, but falls through it. Penelope runs after him, showing she is concerned for his safety. But she slips and falls on top of Slime. She quickly asks if he's okay, and he replies yes.
Probability of Canon Edit
The probability of canon is 90% because the two hit off after Slime healed her, and they looked like they really liked each other.This is enough proof to say it is 90% Canon which is pretty cute.