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.

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.