Why is my sand and rocks in a saltwater tank keep getting rust colored?

I have a 90 gal. saltwater tank that has been set up for 4 months . It got a rust color algae all over the sand and rocks then it turned a velvet black. I treated it for red slime which seemed to take care of it. But now every time I do a water change the rust color comes back with a vengeance. What is this and how can I prevent it. Thank you in advance for your advice.