Ruby overlay in photoshop

No idea where to put this but it’s driving me insane. I’m trying to view a ruby overlay on a layer mask, and everywhere I read about the shortcut is said to be “\” (with the mask selected).

On my apple wireless keyboard I need to press shift+alt+7 to display a backslash. Nothing happens when I do this (forward slash doesn’t work either).

How in holy hell do I display a ruby overlay?

Thanks a million.

Looks like I can go to channels and check “mask” there. Is that the ruby overlay?