No interactions present.
Looks like a geen algae covering the rocks. I think you would need a microscope and a very good key to identify the species.
.....is a misspelling - not deliberate (I'd correct it if I could) ; but it is suitable because, this week, I am blitzing the greens.
Murdo's ( Syrphus) ID is fine but it did not complete the panel and in those cases, usually negates the post (I am informed)
This is almost certainly an Ulva but most certainly NOT U.lactuca (Sea Lettuce) which is why Murdo came and William left his comment.
The Ulvales is a large group of green seaweeds that have had quite a bit of attention in the last ten years and appears to be still undergoing change (or at least investigations). We cannot be absolutely certain it isn't from a group outside the Ulvales but still within the Green Seaweeds (Chlorophyta) of course.
So my suggested ID is genuine and up for agreements (which are few and far between in Seaweeds!)
You'd have thought that EoL (Encyclopaedia of Life) would have more on this group than over 400 random photos - see http://eol.org/pages/4047/overview
I have to hand the best reference book currently available♥
♥ http://www.nhbs.com/title/157360/green-seaweeds-of-britain-and-ireland now already over 6 years old
THREE MARINE PROJECTS
Lat/Lng: 55.07002780978, -1.4539044207611
OS grid ref: NZ349751