You can turn off the slider text through Appearance > Themes > Customize > Graphene: General > Slider, and tick the option that says “Disable slider caption”.
However, looking at your site, those slider text do give good context on what the photo is about. Perhaps instead of turning of the captions, you might want to shorten the captions a bit so they won’t cover the image.
You can create your own captions that are separate from the main body of the the post via the “Excerpt” field for the posts.