is there a plugin or code for moderators to put a comment in a post, that can only be seen/read by moderators and administrators?
You can use the moderator tools to unapprove the posts.
No, I mean in a vissable thread.
So a comment in a user post, and only the comment is hidden for users, and not for mods an admins. Users wil only see their own post
Then you can make a separate forum for it and in the forum's setting, you can check option called,
"Can only view their own threads?".
No, not what I mean.
If a moderator here would make an edit in my post, and his edit would not be shown to me, only visible for moderators and admins of this forum.
Or maybe how i can make a spoilercode, and the spoiler can only be clicked and read by admin or mod