I have looked into it.

The website:

https://www.alltrails.com/trail/us/texas/morningstar-boardwalk/photos

Has a higher resolution image, and to me, it looks most like a recycled tire material, similar to shingles. Since that isn’t really an available option, I suppose the closest thing is paving_stones. The description (https://taginfo.openstreetmap.org/tags/surface%3Dpaving_stones) describes it as

“A surface paved with artificial blocks (block pavers, bricks) or natural stones (flagstones), with a flat top.”

Which to me can cover rubber pavers, similar to the ones seen here, just without the slots:

https://www.menards.com/main/building-materials/landscaping-materials/pavers-patio-blocks/groundsmart-16-dual-sided-rubber-paver/gspv16gy60/p-1444436033321-c-5786.htm

And therefore I believe rubber pavers are the material used in this area of the boardwalk.

I will close this comment, as it is already marked as pavers.