If a non-Horror creature with a slime counter on it somehow becomes a Horror, the effect making it 2/2 and making it lose its abilities ends. Its base power and toughness will revert to whatever it was before that effect started applying, although other effects that modify power and/or toughness, such as ones created by Auras, counters, and the like may still apply. It'll also have whatever abilities it had before it lost them.

Sludge Monster's last ability affects all non-Horror creatures with slime counters on them, not just creatures that had slime counters put on them with Sludge Monster's first ability.