Sorry, but there is no class or ID to add the custom icon for that text. It was before http://prntscr.com/o9078f but not now http://prntscr.com/oke6y9 They are using simple p tag to display the message now. I have changed custom code from .product-view-mask3 .footer-product>.wcmp_add_to_cart_message:before
to .product-view-mask3 .footer-product>p:before
but keep in mind that if that text does not have any custom class and we use just p tag to apply styles then if any other p tag will be displayed in the product footer it will also have that icon.