Gray Character Health Bar?
 

One of my players has had a gray health bar in HLO for weeks now, and they say they don't know how to fix it. I'm not sure if this is a bug or a setting, and haven't been able to find a setting to change it myself. Does anyone have any information on this, and if it's a bug?

If the health bar is showing as grey colored the most likely culprit is a configuration in the Settings Navigation Tab.

Have them Open the Full Character within the scope of the Campaign > Settings > *Scroll Down* > View - Display Settings
Here are the configuration options on how much information to "share" about your visual HP Pool. A GM of any given game should still be able to see how much is missing but that info is hidden from other Players.

This can be used by GMs and Players to "throttle" the granularity of how visible HP Damage is on other Characters in the Stage. It sounds like this player has the "No HP Information setting enabled for the Character.

I hope this helps!

I am going to assume you mean "YOU" the DM see a gray health bar on a PC when they are on the stage. If that is correct than that player has turned on the Setting option "View Current HP".