Hmm, I’m not seeing an easy fix here – I’m actually having a tough time seeing how you accomplished this! I’m on a mini vacation over in Vancouver on a laptop, I’m spoiled at home with multiple monitors and all my development tools! Hard to debug on this thing.
Since you created a custom menu, how about removing the default arrow (hide it using CSS), then add your own arrow inside your custom walker (you can use the Font Awesome caret down icon). Let me know 🙂