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The plugin can definitely override the string ‘optional’ in a standard WooCommerce checkout (which is what that looks like).
You’ll need to find the original string in the plugin or theme code. You can check the FAQ ) or this article (http://plugins.leewillis.co.uk/doc_post/replacing-wordpress-strings-context/) for some examples of what you’re looking for.
Alternatively, the Pro version of the plugin provides a String Discovery mode which means you can search for strings on your site via autocomplete suggestions.
That said, from memory, WooCommerce doesn’t include the brackets in the translatable element, so you’d end up with ‘()’ instead, so I’d suggest looking at an alternate solution – probably hiding with CSS.