You can make your posts only visible to your followers by doing the following two things:
1. Lock your account so that you have to approve who follows you (more info: https://fedi.tips/restricting-who-can-follow-you-in-mastodon/)
2. Set your default posting visibility to followers-only (more info: https://fedi.tips/who-can-see-my-posts-in-mastodon-how-do-i-send-dms-in-mastodon/)
If you do both these things, only your followers will see your posts and only people you approve will be able to follow you.
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All GNU social JP content and data are available under the Creative Commons Attribution 3.0 license.