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I am amazed that so many have agreed. This one, indistinct photo is all we have. Whilst it is the most common Cladophora, I assume that some of the others have similar features.
I have agreed because it is very likely (is that the right spirit?); the picture shows some of the features attributed to rupestris, particularly its light colouring (lighter out, darker in the water) and growing habit. I might be tempted to remove my agreement because it is really asking a lot to ID a green weed in a rock-pool from a single, slightly fuzzy picture. For those who will now see this comment please visit the other Cladophora posts and add support or a comment - it all helps others who may find this prolific marine alga and wish to ID it.
Later.. I have returned to remove my agreement. Nothing vindictive, nor is it a snub to those who have agreed. It is just not possible, via a single view and no description, to separate this from others. Only THREE are now listed in the most recent publication♦ . More though, the whole genera has been thoroughly upended in Green Seaweeds of Britain and Ireland ♥
I have recently added a new post http://www.ispotnature.org/node/682822
Lat/Lng: 50.6115, -2.1318
OS grid ref: SY907790
Seaweed found on a rocky beach in Dorset.