how to get category`s slug in WP_Query loop?

get_the_category() retrieves WP_Term objects for all the categories assigned to the post. You can combine that with wp_list_pluck() to just get the slugs:

$categories = get_the_terms( null, 'project-category' );
$category_slugs = wp_list_pluck( $categories, 'slug' );

$category_slugs will now be an array of slugs for the categories of the post.

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