Gloeocapsa magma

The Asphalt Roofing Manufacturers Association names Gloeocapsa magma with a hedge: probably the most prevalent of several algae species that contribute to discoloration, not the only one and not a certainty.

GAF's current technical bulletin names the same organism differently: tiny cyanobacteria, commonly known as blue-green algae. UF/IFAS Extension's EDIS explains what that word changes: cyanobacteria are true bacteria, neither plant nor fungus, while an alga is one of a large group of photosynthetic organisms. The U.S. Environmental Protection Agency uses both names at once, describing cyanobacteria, also known as blue-green algae, as microscopic organisms that use sunlight to produce their own food. Roofing documents use the word algae; biology files the organism with the bacteria.

How algae differs from mold on a roof is its own answer: algae versus mold on a roof.

ARMA's algae and moss bulletin explains why the discoloration is dark: the most common type, gloeocapsa magma or blue-green algae, protects itself from ultraviolet rays by producing a dark pigmented sheath. GAF says the same of the dark color: a protective sheath the organism forms against UV rays.

ARMA describes the sequence: discoloration typically begins as small spots, then transforms into streaks on the roof plane, and by the time it is visible it has likely been present for several months or longer. On shingle roofs, GAF's bulletin adds that the stains usually cover more area on the lower section of the roof and taper as they reach the ridge. How the streaks look on tile and shingle is set out in the roof cleaning page's section on black streaks.

Why a roof gets the streaks at all is answered at why roofs get black streaks.

ARMA writes that algae discoloration of roofs is most widespread in the Gulf States and along the Northwest and Eastern Seaboards, that it is not confined to those regions, and that it occurs especially where conditions are warm and humid. Florida is a Gulf State; ARMA names no finer region than that.

The word algae covers more ground than this one organism. The glossary entry for algae sets Gloeocapsa magma inside that wider word.

On an asphalt shingle roof, cleaning instructions are the shingle manufacturer's to give, and this page states none.