Two most common reason why SPS stn. Wrong water parameter and pest or disease infection.
Suggest you get a reliable test kit and test for your tank water parameter such as KH level. If possible test for ca and mg as well. Salinity should also be checked with a working refractometer.
Try to recall if you have done anything new to your tank ( such as adding carbon, additives etc ), if so, preferable stop adding if possible for the mean time.
The best possible solution to solve the issue is to do gradual water change with a reliable salt mix...say about 10-25% depends on the severity of the STN. You might need to perform water change for two to three times so as to bring the water parameters back to a normal level.
There are many reasons as to why your sps is having stn, if you find out the water parameters, you can narrow down the cause and solve the issue more effectively.