We often recommend restarting your device if you are having this issue as it's not directly related to Focusmate but seems to be an issue with the camera driver on Mac.

But if you don't want to reboot, fortunately, thanks to a helpful Focusmate user named Sunny, we've learned there is another trick that seems to fix the problem immediately for some people using a command line force quit, and does not require a reboot:

Learn more from OSXDaily here