1) Decomposing agent 2) Something that rots 
• (n.) A causer of decay.
De·cay'er noun A causer of decay. [ R.]
decayer 1. The destruction or decomposition of organic matter as a result of bacterial or fungal action; rot. 2. Causing rotted matter which becomes decomposed; such as, vegetation that was decaying. 3. In physics, radioactive decay. 4. In aerospace, the decrease in orbital altitude of an artificial satellite as a result of conditions; such as, ...