Ulothrix flacca is a small filamentous marine algae.

This species grows as small tufts of green unbranched filaments growing to no more than 10 cm long forming woolly masses.

Sometimes difficult to distinguish Ulothrix from Urospora.

Recorded from Scandinavia south to Portugal, in the Mediterranean and the Black Sea.

[2] It is found on rocks, stones and epiphytically on other algae.