Use any file manager app to create a blank text file entitled ".nomedia" (make sure to have the period in front) in the folder you want Google Play Music and other media apps to ignore.
I use
Total Commander as my file manager. To create a new file there, I long press on the folder where I want to create a file (or in your case, the folder that you want Play Music to ignore), and a menu pops up that includes "Create Text File." Then I can simply save that file and call it ".nomedia"