Ammonia?

I set my reef tank up last Friday using uncured aquacultured rock. Two days ago I did my 1st water test for ammonia, it was 1.5. Yesterday I tested again and it was 0.5. After the test I brushed off some whitish looking stuff that had formed overnight, it wasn't everywhere just some here and some there. So, this morning I do a test and the ammonia is back up to 1.5, is that because I may have brushed off some dead stuff into the water? I'm planning on doing a small water change this coming Friday.