Flexslider change caption style
That is going to be dependent on the name of the field you used for that field.  If you inspect, using your browser's inspection tool (right click on the element and choose "inspect element") you can search the DOM for the appropriate div to use to style the field as a whole.  

Most likely the field name you are looking for will be similar to:

.field-name-field-description


It should use that naming convention with the machine name of the field at the end, with the spaces as underscores.

This will not exist in the flexslider.css file by default.  You will have to add this.  I would recommend that you add this to your theme css rather than the flexslider's css.

Try adding these attributes to your field's selector initially to get a good idea of where it sits on your page relative to the flexslider itself:

float: left;
width: 100%;
background: red;