You might not necessarily need more rock. That all depends on the porosity of your rock. 50lbs of fiji isn't much. I recently bought 50lbs. of rock for my 125 that nearly takes up 3/4 of the tank. I had 70lbs of fiji/tonga already and I ended up putting a good 20lbs of the fiji in my sump. Also, for your bio load right now, that much rock would be enough. If it is a fairly heavy rock, before you start adding anything else, get more cured rock.
You do need more flow. For a 75g, 500gph is a drop in the bucket. I had 2 k3's in my 75g which is 1700gph plus another 300-400gph for the return pump. I'd say get your flow up and cut your lights for a couple days along with a couple 25% water changes. If it is some sort of algae bloom, this should get rid of it. You need to get your parameters under control though or it will probably come back.
If your rock was uncured when you bought it, I'd also say your tank is still cycling. A month is a pretty good amount of time for curing, but that all depends on how you're doing it. If it was cured, then you had a major spike in ammonia from something which died or you feed too much.