Questions,questions sooo many questions!

[Rakahrd]

My 55+sump tank
1 is there such a thing as change too much water or too often? my nitrates are hovering around 5.0 PPM even after a 10% water change. nitrites are 0, ammonia is 0,PH is at 8.4
2. aded my clean up crew and the hair algae is almost all cleaned up on the rocks but it looks like a black fuzz is creeping over some of them. good or bad what to do?
3. corals when do you know its safe to try adding corrals my tank is about 2 months old has 8 small fish and the clean up crew of 25 bluelegs 10 cerreth snails, 20 tiny cerreth snails, 12 Narcissus snails ,cant find my small nudibranch or emerald crabs(2)

 

[Tahoe61]

My 55+sump tank
1 is there such a thing as change too much water or too often? my nitrates are hovering around 5.0 PPM even after a 10% water change. nitrites are 0, ammonia is 0,PH is at 8.4
2. aded my clean up crew and the hair algae is almost all cleaned up on the rocks but it looks like a black fuzz is creeping over some of them. good or bad what to do?
3. corals when do you know its safe to try adding corrals my tank is about 2 months old has 8 small fish and the clean up crew of 25 bluelegs 10 cerreth snails, 20 tiny cerreth snails, 12 Narcissus snails ,cant find my small nudibranch or emerald crabs(2)

I would not give a nitrate value of 5 ppm in a new set up a second thought, no intervention warranted.
I suspect your new algae issue is Cyanobacteria, need an image though.
You can add hardy soft corals now.