Is there a way to password protect a Desktop Folder in Yosemite

That folder cannot be modified because it is required by OS X. And, there is no point in password protecting that folder. It's the Desktop folder. If you have items on the Desktop you don't want accessed, then move them elsewhere on your computer into a separate folder that you can protect with a password.

You need to find a third-party utility that does that. You should find one at macupdate.com or at the App Store. In order to use OS X you would need to create an encrypted disc image using Disk Utility, then store your folder in the disc image. That way only you can mount the disc image and access the data.

How to create a password-protected (encrypted) disk image