Photo of a Fedi tips post that says: The official Mastodon Android app does not have hashtag following. If you're using Android, I would strongly advise using the third party apps Tusky or Fedilab instead, which do support hashtag following and have a lot more features besides. Alternatively you can use the website on your phone, it works well too and gets all the latest features first.
https://files.mastodon.social/media_attachments/files/110/652/598/072/194/764/original/4792777bcf6a9f95.jpg