In order to avoid desiccation by the drying Sun at low tide - that literally bakes the rocks sufficiently hot as to make it uncomfortable to touch them with the bare hand - these seaweed tend to occupy the multiple furrows lacing the flat surface of this beach rock, which capture enough remnant water to help tide over the seaweed during the long periods of low tide.
A dangerous level of drying out can occur on the seashore through a combination of Sun and wind. These images must not however give the impression that seaweed are incapable of attaching to, and surviving on, bare rock that DOES dry out, for I have witnessed other seaweed species on other sectors of the Andhra coastline do precisely that, they able to tolerate desiccation.
I am not exactly sure what this seaweed species is - but it must be either Enteromorpha compressa or Enteromorpha intestinalis.
March 1, 2020, Yarada Beach, Andhra Pradesh.