How can/should I indicate that it's okay to use open access content in a paper?
Almost all CC licenses include an attribution clause. As of the 4.0 version, this means:


  Attribution — You must give appropriate credit, provide a link to the license, and indicate if changes were made. You may do so in any reasonable manner, but not in any way that suggests the licensor endorses you or your use.


Clicking the "appropriate credit" shows some more information:


  If supplied, you must provide the name of the creator and attribution parties, a copyright notice, a license notice, a disclaimer notice, and a link to the material. CC licenses prior to Version 4.0 also require you to provide the title of the material if supplied, and may have other slight differences.


So the easiest way to satisfy this is, indeed, to say


  Figure from reference [x], used under the terms of the Creative Commons Attribution 4.0 license.


Where your reference should contain the rest of the information, including a link. If you're pressed for space, you can abbreviate the license (CC BY 4.0), or put the text in a footnote.