I believe greencode was trying to hide the image title when you hover over it but I stumbled upon this thread because I was trying to hide the caption pop-up on hover. After some trial and error I got what I needed by adding this code at the bottom of my themes CSS file.
.tiled-gallery-caption {
display:none !important;
}
With this code the captions no longer pop up when a cursor hovers over an image, but the caption will appear once you view the images in the gallery carousel.
(Note: I’m not sure if this is important but I am using the Tiled Galleries Without Jetpack plugin instead of the full Jetpack plugin because the only features I need are the tiled galleries and carousel.)