Here's another view of a branched alga showing a similar effect:
We knew it couldn't be frost, and when we took a closer look, we were impressed:
These are extensive bryozoan colonies covering the algae. In the photo above, the bryozoan Membranipora has grown around the branches of Neorhodomela larix (a common intertidal seaweed).
I've posted about bryozoans a few times, but to see what these beautiful marine invertebrate colonies look like when submerged and viewed under a microscope, check out the post from 27 January 2015, especially the 5th and 6th pictures.
I'm not sure why the bryozoans are so extensive this year, but my guess is that they've done very well with the warm ocean temperatures during the last two years.