This alga forms patches or larger sheets of thallus less than 0.5 mm (0.02 in) thick tightly attached to the substrate.

The surface is smooth and flat apart from slight mounds indicating the presence of conceptacles (specialized cavities containing the reproductive organs).

[3] Like other algae, Hildenbrandia rubra is an autotroph, using photosynthesis to convert carbon dioxide and water into the organic compounds needed for maintenance and growth.

[5] Hildenbrandia rubra is a very tolerant species, able to grow in a wide variety of salinities, temperatures and light conditions.

[3] In an experiment it was exposed successively to freezing, desiccation, low salinity and high temperature, but the rate of photosynthesis showed no long term variation; in fact it was the most tolerant species of algae of any of those tested.