I'm not sure it works... I think all the Geoscape icons should be simplified to the max. Right now they look like thumbnails. You can see the purple dot but that's it. You cant really tell what sort of a symbol is located in the center. That Kerrblade is barely visible but changing the background color wont help.
I'd suggest getting rid of all the details from the texture of the center element of the icon. Use large shapes and a single color only. It will make the icons easy to distinguish from one another while keeping the original design of the symbols intact.
Sitters - sorry for messing with your pics without permission but that was the best way for me to illustrate my thoughts.
Here's an alternative design of the terror symbol.
and here's why I think it would work better than the previous one:
Even though the icon is small you can still tell that it's indicating a terror strike. The Kerrblades are still visible although they look like an 'x'. That's a minor issue though... and to tell you the truth, the purple color of the icon itself would do the trick as well.