'algae-friendly' originates from Modern English compounding the noun 'algae' and the adjective 'friendly'. 'algae' is the plural form of Latin 'alga' meaning 'seaweed', and 'friendly' comes from Old English 'frēondlic' (from 'frēond', 'friend'), meaning 'characteristic of a friend'.

The element 'algae' derives from Medieval/Modern Latin 'alga' ('seaweed'), adopted into scientific English as 'alga' with plural 'algae'. 'friendly' developed from Old English 'frēondlic' through Middle English into Modern English 'friendly'. The compound 'algae-friendly' is a modern English formation combining these two elements to describe things compatible with or beneficial to algae.

Initially 'alga' simply meant 'seaweed' and 'friendly' meant 'like a friend' or 'favorable'; over time the compound came to mean 'favorable to or not harmful to algae' in ecological and product-descriptor contexts.