'algaecide-treated' is a compound formed from 'algaecide' + 'treated'. 'algaecide' originates from Latin 'alga' where it meant 'seaweed' (through Modern Latin/English usage) and the suffix '-cide' ultimately from Latin 'caedere' meaning 'to kill'. 'treated' originates from Old French 'traiter', from Latin 'tractare' meaning 'to handle or manage'.

'algaecide' arose in English by combining 'algae' (from Latin 'alga') with the productive suffix '-cide' in the 19th–20th centuries; 'treated' came into English via Old French 'traiter' and Middle English forms (e.g. 'treaten'/'treat'), becoming the modern past-participle/adjective 'treated'. These elements combined in modern English usage to form the compound adjective 'algaecide-treated'.

Initially, 'algaecide' referred specifically to a substance that kills algae and 'treated' simply meant 'handled' or 'subjected to a process'; combined, the modern meaning is specialized to 'having been subjected to treatment with an algaecide to prevent or remove algae.'