'algae-unfriendly' originates from Modern English as a compound of 'algae' and 'unfriendly'. 'algae' comes from Latin 'alga' (from earlier sources ultimately associated with seaweed), and 'unfriendly' is built from the negative prefix 'un-' plus 'friendly' (Old English 'frēond' meaning 'friend' with adjectival suffixes).

'algae' entered English from Latin 'alga' (seaweed), itself linked to Greek/Latin maritime vocabulary; 'unfriendly' developed from Old English roots ('un-' + 'frēond') and later took the modern adjectival form with '-ly'. In contemporary English these elements were compounded to form the descriptive phrase 'algae-unfriendly'.

Initially the parts meant 'seaweed' ('algae') and 'not friendly' ('unfriendly'); combined in Modern English the compound came to mean 'hostile or not conducive to algae', used especially for environments, chemicals, or coatings.