Several names share this combination and are defined under the rules of the botanical code. The species name Ulva fusca might be an homonymy case.
The name Ulva fusca is a slight homonymy case. This name is used according to the rules of several different nomenclatural codes.

Ulva fusca Huds.

Botany

Ulva fusca Postels & Rupr.

Botany

Ulva fusca Postels & Rupr.

Zoology