When I had Gracilaria, my preferred method was to find a small piece of rubble rock with a good sized hole in it and string a piece thru. As it grew and started branching out, both sides were bulky enough to not slip thru the hole and stay secure.
I also would take a big clump and use a small zip tie to secure it to a rubble rock. I’d then bury as much of the rock in the sand that I could to keep it in place. The Gracilaria would then bulk up to the point where the zip tie was not that noticeable and look good.
I’ve seen others recommend fishing line as it is secure and not that visible at all, but it never worked well for me and my clumsy fingers.