I'm not 100% convinced of my attempt of an ID, but I think I've stumbled across a
Stichodactyla gigantea today.
The tentacles vibrate like crazy, the oral disk tends to fold up instead of lay flat, and the verrucae are not only present and visible, but they are purple (
S. haddoni tend to have verrucae that are not a contrasting colour). This guy is TINY, about 6", so hopefully time will tell for sure. It is a very rigorous specimen (attaching to substrate instantly and robustly), which I hope bodes well for its health. It has already captured a few stray pieces of mysis when I fed my fish.
Wish me luck!