Remove "Ads by Games Crystal" malware from Chrome on a Mac
Although I was hoping to find a more surgical solution, I ultimately settled on simply resetting Chrome, by following the instructions here:


On your computer, open Chrome.
At the top right, click More > Settings.
At the bottom, click Show advanced settings.
Under the section "Reset settings,” click Reset settings.
In the box that appears, click Reset. ​


So far, no additional Games Crystal ads have displayed, so hopefully this took care of it.