Thanks for reaching out.
It's a post slider from Ethos stack, you can configure or toggle it off in Theme Options > Ethos. Please check this https://theme.co/apex/forum/t/customizer-ethos-options/131 (post slider) for more information. It's a bit old but still same options in Theme Options instead of Customizer.
As for the post slider button, it has its own styling to match the Ethos post slider layout and it's meant to be like that. It only appears like that since you only changed the background color and border radius through custom CSS. And it's not a button, it's just a layer within the link as "read more". Are you referring to the home page's button instead?