The National Geographic Style Manual at http://stylemanual.ngs.org/home/A/alga-algae has (selected extracts):
Alga is the singular noun and requires a singular verb
Algae is the plural form and requires a plural verb:
When the algae die, they fall to the bottom.
Occasionally algae may be considered a collective [mass] noun and treated as singular:
A map shows how algae periodically blooms in the bay.
Look! There it has drifted under the bridges, and out into the ocean, where it hovers over the reef.
However, there is an inconsistency in their approach; the example they give for the use of the singular noun is:
A colony of Licmophora fans itself across another alga, Bonnemaisonia. This should surely be another mass-noun usage, but they're using the singular, alga. Their example should read: A colony of Licmophora fans itself across a colony of another alga, Bonnemaisonia.
The trouble here is that alga is rarely considered as a single particle - more often as a single strain. One virion or one bacterium are reasonably commonly considered.