I am both unicellular and multicellular depending on how old I am. When I am getting ready to reproduce, my cells join together to make a slippery, slithering mass that is bright orange. When mature, I dry out and release millions of spores. I eat by digesting dead leaves and absorbing the nutrients.

What kingdom do I belong in?

Kingdom Protista.

Explanation:

This is a perfect behaviour of a slime mold which is known to be of the kingdom protista. Slime molds are usually found in lawns, and on the forest floor, deciduous logs and soil too.

Protists are made up of eucaryotic cells in which the cell structure is complex with a well defined nucleus. Some of them move around with their flagella or by forming pseudopodia also known as their false sheet.