I do one water change once a week
my water is perfect....
Not to try to sound nasty, but this post tells us nothing at all. What size tank do you have? How much water do you change? Do you use RO/DI water to mix your salt? What brand of salt do you use? etc etc etc.
Saying your water is perfect also tells us nothing. What are the exact readings your getting? What may seem "perfect" to you may indicate a major problem to us. You also may not be testing for critical parameters.
There is obviously some sort of problem, or you would not be having problems with your frogspawn. Of course the problem could be an unhealthy coral to start with, but more likely there is something about your tank that it doesn't like. For example, you may behaving "chemical warfare" between two corals, and your frogspawn is loosing. You could have introduced something toxic to the system. Some LFSs run copper in there fish section to keep parasites down. If you add that water to your tank, even in small amounts, you can kill corals.