'algae-covered' is a compound formed from the noun 'algae' and the past participle 'covered'. 'algae' comes from New Latin 'alga' (from Latin 'alga'), originally from Greek 'algē' meaning 'seaweed'; 'covered' is the past participle of 'cover', from Old French 'covrir' (Modern French 'couvrir') ultimately from Latin 'cooperire' ('co-' + 'operire').

'algae' entered English from New Latin 'alga' (itself from Latin and Greek) and became the modern English noun 'algae'; 'cover' developed from Latin 'cooperire' to Old French 'covrir' and Middle English 'cover(en)', with 'covered' as its past participle; the compound 'algae-covered' is a modern English formation combining these elements to describe something that is covered with algae.

The components originally meant 'seaweed' ('algae') and 'to place over or hide' ('cover'); combined in modern usage they specifically mean 'having a surface layer of algae' without major shift in the component meanings.