It’s a fairly standard procedure for reducing spam. Takes about 10-20 minutes of reading and interacting with the forum to get “Basic”, which is when you can post images and links. It took me:
This is an option that you can uncheck. I believe it is supposed to show when you’re in combat.
Interface Options > Hud display > Miscellaneous > Enable combat display
I never even noticed that shadow before, but now that i looked, it’s constantly there.
If it’s supposed to show just in combat, it really seems bugged.
Nice to have the option to toggle it off, though.