Porphyra cannot be separated from Pyropia without attention to fine detail.
To separate the various species of Porphyra requires careful attention to the morphology and location habitat and usually requires microscopic examination.
Strictly speaking it is difficult to be certain of species without Microscopy.
P.linearis in one of the easier species and often grows on vertical surfaces, such as concrete gabions and slipways. it is at home in shallow horizontal surfaces on sand but is almost always attached to rock or stones.
It is narrow and long but not always a red as seen.. In the UK it is called Winter Laver because it does specially well in the dark months.

See https://www.inaturalist.org/observations/9319860