Hi there,
Thanks for using Poseidon.
I’m sorry to tell you that WordPress does not support different featured images by default.
However, you can deactivate the featured images to be displayed on single posts in the theme settings (Appearance > Customize > Theme Options). Then only the slider shows the featured image. You can show a normal image within the post content than, which can be different than the slider image. That might be a good workaround.
Other than that, you can also try to extend the WordPress Core functionality with extra plugins. For example: https://www.ads-software.com/plugins/multiple-featured-images/
Best,
Thomas