Microscopic plants growing in decaying matter are called decomposers. While the term "decomposers" is a broad category, the specific types of microscopic plants you're likely thinking of are:

* Fungi: These are heterotrophic organisms that break down organic matter for food. They play a crucial role in recycling nutrients back into the ecosystem.

* Bacteria: These single-celled organisms are also heterotrophic and play a significant role in decomposition, particularly in breaking down complex organic compounds.

Important Note: While some algae can grow in decaying matter, they are typically photosynthetic organisms and don't play a major role in decomposition like fungi and bacteria.