Hello,
This is not the ALT text. This is the link to the image with additional data that comes from Joomla (on version 4.0 and later). I don't think this has any effect on SEO.
If you insist on removing the part of the link after #, you will need to disable the editor in the global configuration, edit the page in plain HTML and remove this part of the link.
Regards,
Milos
