I tried the following so far, I got rid of the initial Caulerpa species, though, because I got tired of constantly having to pluck/remove (my tank is small).
C. taxifolia and C. prolifera grew well with their roots on the sand (Ocean Direct Live Sand, rather fine). C. prolifera has that nice, waving, seagrass-like look you might go for
C. brachypus was a PITA. Not only did it grow on/over absolutely everything (sand, rocks, whatever it could grip with its extremely fast-growing runners), but also it constantly shed small parts/leaves?, which either took hold in other parts of the tank, or clogged my Koralia pump (I had to clean the pump sieve every 2 days!!!)
A recommended algae because it does not go sexual like Caulerpa, and because it does not attach to anything, is Chaetomorpha. Little critters like Gammarids etc. absolutely love this algae, and usually remain there when not checking the sand/rocks for food.
What I like best are 2 species of red macro algae I was not told the names of, but they do not attach to anything either, so they do not become a pest.
They do well when being gently shoved into holes in the rock with the lowest part of their stems.
When I tried clamping a bunch of that algae down on the sand floor (weighing the lowest part of the stem down with a stone), that bunch died. So I would not recommend trying that.
Those were just a hobbyist´s experiences. For scientific background, please stick to Octoshark´s advice
). Then, on the other hand, you have stuff like caulerpa that can do fine in sand. So substrate issues will depend upon your intended species of macro.
