Nameplates

Press V to toggle enemy nameplates.

Press Shift+V to toggle allied nameplates.

Press Ctrl+V to toggle both.

If you have rebound the keybinds to something else, you can also toggle them in ESC → Interface → Names

Quote directly from BigWigs discord:

For the people asking about a global disable option for the new nameplates feature, no such option exists, it will be added in future. If you wish to disable it, you can do so on a per-ability basis in the dungeon trash settings.

This means that the font you're using does not support that symbol.

/plater → Modding → M+ Colored Mobs

/plater → Modding → Disable ToxiUI Abbrev Names