How to remove gray background and loading animation

Copy this in your theme custom CSS field (if presents) or in your style.css file:

.crellyslider > .cs-preloader { background-color: none; }

This should be:

.crellyslider > .cs-preloader { background-color: transparent; }

as as none is not a valid background-color value. Please take look at the following Stack Overflow discussion: https://stackoverflow.com/questions/8739665/is-background-colornone-valid-css