Somebody is attacking your star. TAKE THE STAR, AND PUT HIM IN THE SUMP--He'll be safe there...
As for the pH. Testing the pH with a test kit is completely useless IMO... The pH swings throughout the day, sometimes more than 0.5 pH in a few hours. If you really want to check the pH, get a digital pH monitor, and you'll be able to see how the pH changes over the course of the day...
You might have a new and unwelcomed addition to your tank--any new rocks?
Or--some baddy is showing himself for the first time...
Any other inverts disappear over the past few months??
Isolate the Star, for his own good...
good luck
Steve