General terms:
* Saprotroph
* Decomposer organism
* Detritivore (specifically for organisms that feed on dead organic matter)
* Scavenger (for animals that feed on carrion)
More specific terms:
* Fungi (a major group of decomposers)
* Bacteria (another major group of decomposers)
* Worm (earthworms and other worms are important decomposers)
* Insects (many insects, such as beetles and flies, decompose organic matter)
The best word to use will depend on the specific context and the level of detail you need.