When it comes to SPS the difference between growing and killing corals can be very minute. The diference between "growing" and "keeping" even less. There are many things at play in a young tank that cannot be controlled or even tested for even when starting with "mature" rocks. Just focus on keeping the water parameters that you can test and control stable, and the rest will work itself out. If you keep the water parameters stable and correct for growing SPS one day out of the blue you are gonna walk up to that tank and wonder when that slimer got so big.