red alga

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of various predominantly tropical marine algae of the division Rhodophyta, characteristically red or reddish in color. Red algae are important as a food source and in reef building.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Any marine alga, of the phylum Rhodophyta, that are mostly reddish in colour