Lichen is a symbiotic decomposer that attachers itself to trees and slowly breaks them down. they obtain nutrients from the tree, allowing them to survive and reproduce. what type of relationship is this?

This type of relationship is called Parasitism 
Parasitism is a type of symbiotic relationship, or a long-term relationship between two species, where one of the organism, the parasite,gains benefits that come at the expense of the host member. Examples of parasitism includes; Fleas or ticks that live on dogs and cats are parasites. In the above question; Lichen is the parasite and the tree is the host. 

Might need you in a few weeks c: