Diatom and Coralline

[DACJ]

My tank has been setup for over 6 months now using live rock and sand from an old system. it has been fine all along up until last couple of weeks. firat the coralline algae seems to have stopped growing on the rocks. and it almost seem like there is a diatom bloom , on the back of the display only....long brown hair algae looking. comes off easily with a scrape. fiah are fine corals are fine anemone still alive. water level all at beat except for about 5ppm nitrate.
10 percent water change every week...any ideas ?

by the way, i should note that my salt mixed got moist and hardened for about half of the bucket which i still used for a while until i finally smashed through yesterday then dump out the hardened salt. now the bucket has normal mix now. BUT . could the usage of moistened salr mixture be the xause?