This could be a whole host of things. You say that your water checks out fine. What's fine? Please list parameters and how long they've been stable at those numbers.
1. How old is the tank?
2. Do you run GFO? How much?
3. Do you run carbon? How much? How often is it changed?
3. What other corals/fish are in the tank?
4. What is your water flow?
5. What is your lighting? Photoperiod?
6. Do you run a skimmer? What skimmer? How often is it cleaned?
7. How often do you feed?
8. What is your water change and maintenance schedule?
9. Are you using RO/DI water for top off and water changes?
10. Can you grow coralline algae?
11. Do you have a good phosphate test kit?
12. Do you vodka/carbon dose
13. What salt do you use?
Aside from some of the above, a couple of the first responses you'll hear are pests like acropora eating flatworms, redbugs, temperature shifts, inadequate flow, alkalinity shifts, salinity shifts (maybe your refractometer needs to be calibrated or your hydrometer needs to be rinsed thoroughly so that the swing arm rides freely).
You'd be surprised as to how easily sps can be lost. You state that this is your first one. There are many answers that need to be learned by the rest of us in order to properly give you an adequate analysis.