'algal-bloom-promoting' is a modern English compound formed from 'algal' + 'bloom' + 'promoting'. 'Algal' derives from Latin 'alga' (from Greek 'alga' / 'αλγη'), originally meaning 'seaweed'; 'bloom' comes from Old English/Middle English 'blōma' (related to Old Norse/Proto-Germanic words for 'flower' or 'flourishing'); 'promoting' comes from Latin 'promovere' (pro- 'forward' + movere 'to move').

'algal' developed as an adjective from 'algae' with the suffix '-al'; 'bloom' retained its sense of rapid flourishing and was applied to algal growth in ecology; 'promote' passed from Latin 'promovere' into Middle English as 'promoten'/'promouten' and modern English 'promote'. These elements combined in contemporary scientific and media usage to form the compound 'algal-bloom-promoting'.

Each component largely kept its original sense (algal = relating to algae; bloom = rapid growth/flourishing; promoting = causing or encouraging). Combined, the compound came to mean specifically 'causing or encouraging algal blooms' as a technical/descriptor phrase in ecology and environmental science.