ScreenMarker is not available for Mac but there are a few alternatives that runs on macOS with similar functionality. The best Mac alternative is Ultimate Pen. It's not free, so if you're looking for a free alternative, you could try FlySketch. If that doesn't suit you, our users have ranked five alternatives to ScreenMarker and three of them are available for Mac so hopefully you can find a suitable replacement.
Ultimate Pen is a software only solution which creates a full screen, transparent window, allowing you to draw on top of most applications. Think of Ultimate Pen as an incredibly easy way to doodle on your computer screen.