Sea gnome,
Sounds like you are doing the right things.
The low pH is a bit distracting, but it is fairly common with hair algae and it should get better as the hair algae receeds. How often do you harvest the calerpa and have you considered boosting the lighting time for the refugium to 24/7. 24/7 lighting should help calerpa grow more often and reduce the chances of it going sexual.
Good luck !
- Victor.