'algal-promoting' originates from Modern English, specifically the compound of 'algal' and 'promoting', where 'algal' is derived from 'alga' (Latin) meaning 'seaweed' and the adjective-forming suffix '-al', and 'promoting' comes from Latin 'promovere' meaning 'to move forward' or 'to advance'.

'alga' was used in Latin to mean 'seaweed' and gave rise to the Modern English 'algae' and adjective 'algal'; 'promovere' passed into Old French as 'promouvoir' and into Middle English as 'promote', eventually forming the present participle 'promoting'; the modern compound 'algal-promoting' is a recent formation in scientific and environmental usage.

Initially related separately to 'seaweed' (from 'alga') and 'moving forward/advancing' (from 'promovere'); over time the compound came to mean specifically 'encouraging the growth or proliferation of algae' in ecological contexts.