Not sure if this helps, but check out:
http://saltcorner.com/sections/guest.../algaepage.htm
Great page for IDing algaes of all types and possible biological controls. The dictyota algaes are listed as #17, and according to the control section the rabbitfishes they list don't work (doliatus not listed though). The naso tangs are the only tangs cited for #17, as well as mithrax crabs and diadema urchin. Sea hare not cited for dictyota either.
Sounds like a tough one to control
Francis