The answer is fungus-like protist.
Since it is eukaryotic, it can be either Protista, Fungi, Plantae and Animalia. In all of these groups, multicellular organisms can be found. But spore reproduction and a motility in some stages of its life cycle, suggest it can be protist. Since it is a decomposer, it is probably some fungus-like protist, probably slime molds.
