`function_exists` should be used after the function name in a theme not before.

    add_action( 'loop_start', 'add_slider' );
    
    function add_slider() {
    
    if ( function_exists( 'soliloquy' ) ) { 
    
    soliloquy( 'slider', 'slug' ); 
    
        }
    
    }

This checks to make sure the plugin is active before outputting a function and/or markup otherwise you might get a error like call to undefined function.