If you are interested in a Masters or Doctoral degree, I highly recommend Boise State University's Educational Technology program. It is completely online and you can take courses specific for adult education. Also, you may want to try searches related to "andragogy" as it is specific to adult education.
EDX offers a variety of courses from well known Universities ,i.e. Harvard, Berkeley, etc. Many of the courses are free or you have the choice to get a certificate or degree for a reasonable fee. https://www.edx.org/course
Athabasca University offers two courses in Adult Education, as part of its Masters of Education: MDDE 613 Adult Education and Lifelong Learning, and MDDE 611: Foundations of Adult Education. I didn't do these as part of my MEd, but studied alongside students teaching English to adults who did. These are conventional distance education courses, not MOOCs: http://www.athabascau.ca/syllabi/mdde/mdde613.php
You can also take a look at Electronic Platform for Adult Learning in Europe (EPALE) which is EU tool: https://ec.europa.eu/epale/en/home-page. Available in many languages