You can style the placeholder text with CSS but not change it.
I found an article below that suggested it could be done with PHP filter so I tried adding them to the “Plus” Action and filter box, but without sucess.
May be @weaver could tell us if these filters should work win the Xtreme Action and filter box and if they need modification
Other article with different suggestions, using a plugin or a child theme:
The code given on the page above placed in the Action and filter box, does successfully changes the whole search box