Some specie of Halimeda grow in sand at fairly deep depths. Unsure how easy it will be for you to find these. Ventricaria bubble algae is usually considered a pest in reef tanks, but it will certainly grow on soft sand in lower flow and lower light. With moderate light Caulerpas and Udotea. I'm certain there are red algae and maybe some brown algae that are appropriate, but I can't think of any that anchor in soft sand right now.