Welcome to Android Central! Are you connecting the phone to your computer before the apps go grey? When you connect the phone via USB to the computer, it may be causing the external SD card to unmount from the phone, so that it can be mounted to the computer as an external drive. Unmounting the card from the phone might essentially break the link for the apps that are installed to SD (remember that these apps are broken up, with the main portion in Internal Storage, and a portion on the SD), and render them inoperable until the next time you reboot.