'algae-reducing' is an English compound formed by combining 'algae' (the plural of 'alga', Neo-Latin for seaweed) and the present participle 'reducing' (from the verb 'reduce').

'algae' comes from New Latin 'alga' (meaning 'seaweed'), while 'reduce' comes from Latin 'reducere' ('re-' + 'ducere', 'to lead back'). In English, 'reduce' passed through Old French/Latin-derived forms and became Middle English 'reduces/reduced', leading to modern 'reduce' and its participle 'reducing'. The compound 'algae-reducing' is a modern English formation using that participle as an adjective.

Individually, 'alga' originally referred to seaweed and 'reducere' originally meant 'to lead back'; in modern English 'reduce' now commonly means 'to make smaller or less', so combined usage yields 'algae-reducing' meaning 'decreasing or inhibiting algae'.