newbie turned 125 gal.long into reef with fish

Hi all. Of you. I've Ben renting posts on here for a Mon. And can,t tell you how much it is helping us with our new journey. We've had our fresh fish for 40 yrs.we re retiring and switching to a SLT tank has been our dream.our tank has been started for 4 weeks now. I'll posst all my Chen levels later today.we make our own to water,with some live rock we got from Petco. All of my reading r where they should be. When should I get an ammonia spike? After adding olive rock we now have a brown slim my looking algae blowing round on live rock. Do you think it is diatoms? Its on my sand in a spot behind rocks. So sorry so long but I want to make our tank the beautiful tank we've waited solo long to do.I a t to take time and do it right, but I am doing something wrong..keep yr posts go in I appreciate them all..deedee