Chaeto died

[whatcaneyedo]

More light could help. I use a 15W 6700K CF to light my nano tanks 2gal fuge for growing caulerpa and it does an alright job but more light would produce better growth for me and might help for you. Testing for phosphate could also yield another answer, without any fish in your tank it could be that your phosphate levels are extremely low or zero.

I've seen a colony of chaeto die before in near zero phosphate while another macro algae species did fine so maybe consider trying something else. Gracilaria is another very popular choice, it has basically all of the same benefits of chaeto and none of problems of caulerpa. Plus most tangs love it.