Yep, the glow means, "You've got aggro". The Aggro (sometimes
mistakenly referred to as threat) Glow is part of the default Tidy
Plates feature set (Threat Plates not required). I'm not sure if
Clean Plates has a UI option to turn it off (It's not one of my
themes, actually), but it's possible to do it in the LUA file. Let me
know if you'd like to go ahead and mess with it.
Of note: Only one theme can be used at a time, so if you DID have
Threat Plates available but inactive, it wouldn't do anything.