The --recursive flag is meant to operate on directories. If you were to pass a glob that matches directories, then these directories would be examined recursively but if you pass a glob that matches files, like you are doing, then --recursive is ineffective. I would suggest not using --recursive with a glob because globs already have the capability to look recursively in subdirectories. You could do:
mocha 'src/app/**/*.tests.js'
This would match all files that match *.tests.js recursively in src/app. Note how I’m using single quotes around the pattern. This is to quote the pattern so that it is passed as-is to Mocha’s globbing code. Otherwise, your shell might interpret it. Some shells, depending on options, will translate ** into * and you won’t get the results you want.