That is core Bluesky content.
What feed are you in? “Following” shouldn’t have any of that, unless you are following someone who posts that stuff, or someone you are following is interacting with it. If that is the case, then maybe some un-follows are called for.
“Discover” and “What’s Hot” and “What’s Hot Classic” are more of the algorithmic type feed most people are used to. Classic may have adult content, but any adult images should be behind an NSFW click thing. The main thing to report is any nudity that is not behind an NSFW click thing.
Definitely block any accounts you don’t want to see. That is the Bluesky way.
ALT just means that there is descriptive text associated with the image. It is an accessibility feature.