Is there a difference between mummified and decayed?

Yes, mummification (as in the ancient Egyptians) is when a body is preserved in such a way that it doesn't decay.

"Mummified" means "preserved from decay" in some way, either natural or artificial. Decay is the overall breakdown or destruction of biological material, and usually destroys soft tissue such as skin. However, mummification means that the skin or outer layer is less affected. This typically occurs through drying, or the application of chemicals that prevent bacterial action.