Lawnmower Blennies are not seahorse safe IME. Blue legged hermits are also more on the risky side, however a lawn mower blennie is capable of killing a seahorse. Often times seahores grow algae on there backs and the chasing of the blennie can be quite stressful.
4 seahares in a 75g tank aren't going to last you very long a all. IME two in a 65g was overkill. JME
Nuissance alage in a new tank is no that uncommon.
Even though your phosphates measure at 0, it is still possible and very likely they are there and being absorbed by the macro and hair algaes.
I'd keep doing what your doing, remember to rinse the mysis for the seahorses and, keep a close eye out. Very often new tanks will experience a nuissance algae phase, just keep on top of it.
BTW IME Turbo's love red macro's. :D