CSS Image Conflict with Hustle

Team WPMU,

I've got a conflict with an icon stored somewhere in Hustle. It only appears when Hustle is activated and the site is responsive (conflict is in responsive menu).

Instead of using the theme's icons, the site is using an icon from /wp-content/plugins/wordpress-popup/assets/css/maps/optins/global/_icons.scss:28

This occurs whether the optin is an email optin or a content one. If Hustle is activated on the site, the menu icons are affected.

You can see this in action on http://nickcolebooks.com and http://leighgendarium.com

How can I fix this?