It’s purple because it’s lifted directly from Warcraft 3 TFT expansion where the Warden has blink. It was just changed so it looks more like a generic human than a night elf.
The shape is of a head. The nose is right below the blinking light, and his mouth is that downward sloping line on the left, and his popped collar is on the bottom right.
Yes, Warcraft humans have some pretty crazy chins that stick out far ahead of their face.
The original icon was a Warden’s eye flashing through her helmet. They very awkwardly redid it when they gave it to mages in WoW to make it look like the classic bearded wizard.