After a few months of having corals, you should start testing for at least cal/alk on a weekly basis. At some point, they'll start using more cal/alk than you can replenish with water changes. I'm pretty sure you're not at that point yet, but testing won't hurt anything.
With the timing of when your hammer went bad, and how quickly it went bad, I really really don't think it's your water parameters. Too often the Euphyllia species of corals get damaged during shipment and it just takes a few weeks to become obvious. They're definitely easy corals to keep, but that assumes you get a healthy one to start with. With how quick it came off the skeleton after you brought it home, it wasn't your water parameters. It was damaged when you bought it, but because you're just starting out you didn't know what to look for. Been there... done that... bought the t-shirt AND lost a few corals in the process!