Hi,
Overlay on the slider is providing the contrast between text and image. You can use this custom css code to remove it.

.slider-box .post-wrap:before { background: none; }

When you remove it, text will not be readable, so you can add this custom css code as well,

.slider-box .post-thumbnail .entry-title a, .post-thumbnail .more-link { color: #000; }
.is-slider-more-link-button-style .post-thumbnail .more-link { border: 2px solid #000; }

Thanks