I guess you're a Druid so it could be your mana percentage. I mean, when you're in cat/bear form your mana doesn't show so it could be an indicator to how much mana you have.
He didn't say it was hidden in combat, just that he hadn't noticed it while he had been in combat. But I guess you're busy with other things during combat to notice such a small thing.
Also, as feral you are at 100% mana pretty much all the time.