'algal' + 'stimulating' originates from Modern English compound formation: 'algal' (from Modern Latin 'alga'/'algae' meaning 'seaweed') combined with 'stimulating' (from Latin 'stimulare' via English 'stimulate'), where 'alga' meant 'seaweed' and 'stimulare' (related to 'stimulus') meant 'to goad or incite'.

'algal-stimulating' is a modern English compound formed by joining the adjective 'algal' (derived from Latin 'alga'/'algae') with the present-participle adjective 'stimulating' (from Latin 'stimulare' → Medieval/Modern English 'stimulate'), producing a descriptive adjective meaning 'promoting algal activity'.

Initially, 'alga' referred specifically to 'seaweed' and 'stimulare' to 'goad or incite'; over time the compound came to mean 'producing conditions that promote algal growth' in ecological and environmental contexts.