When creating your Fusion Slide, the description says “Select this option if you would like a semi-transparent background behind your heading”. This shows you how to adjust the level of transparency or change it to opaque.
Avada > Theme options > Custom CSS
For 80% opacity
.fusion-title-sc-wrapper{background-color: rgba(255,255,255,0.8) !important}
or for 100% background opacity
.fusion-title-sc-wrapper{background-color: #ffffff !important}
Change to preferred color and opacity of course.