I haven't had a similar problem, but it seems very reasonable to me.
The Caulerpa was using the phosphate in the tank. When it went sexual, growth of the Caulerpa stopped and it left the hair algae as the main user for the phosphate. Utilization of the phosphate by the Caulerpa started again as soon as it recovered.
I think you need to take a look at what you are doing for phosphate control in your tank, especially in the event when the Caulerpa goes sexual again.
Gary
Gary
125 SPS, 75 gal. LPS/softie reef, 9 gal. Nano