A sedentary sea anemone gets attached to the shell lining of hermit crab. The association is

A sedentary sea anemone gets attached to the shell lining of hermit crab. They have a symbiotic relationship where both creatures benefit from living together. The sea anemone eats scraps of food that the hermit crab releases as she eats, and the hermit crab is protected from predators like the octopus by the painful sting of the sea anemone's tentacles.

Ref: https://seatalespublishing.com/uncategorized/a-hermit-crab-and-her-sea-anemones/

The symbiotic relation between the hermit crab and the sea anemone can be termed protocooperation.

Mutualism is a biotic interaction between two organisms of different species where both partners are benefited. The relation is obligatory. 

Protocooperation is a type of interaction between two organisms of different species in which both are benefited but the relation is not obligatory.