What type of ecological relationship is this?

Commensalism is a long-term biological interaction in which members of one species gain benefits while these of the other species neither benefit nor are harmed. This means these species may obtain nutrients, shelter, support, or locomotion from the host species, which is substantially unaffected.

The communal species is often between a larger host and a smaller commensal. Commensalisms change in strength and duration from intimate, long-lived symbioses, to brief, weak interactions through intermediaries. Another example of commensalism is when an anemone attaches to hermit crab shells.