I was writing poetry (trying to write) under the tag MastoPrompt and I had no idea what that meant either. Now that I think about, perhaps the "masto" part is for Mastodon, which makes sense for "MastoArt", but "MastoPrompt" for poetry? 
Thank you for the response.
@Pat It seems to be a common hashtag for art on mastodon. Click on it to see other examples.