I don't think there is such a thing as offset_x.

Can't you just change pos_x ? Or is the problem that you want it to move out of the window and not be displayed any more?
The other thing you can try (not tested) is to use a render_target and render onto a (possibly transparent, if you don't want any additional images) panel. Then display that panel instead of displaying the text directly on the screen. It should have the same effect, and if you only do it for a text, it shouldn't be much slower (but don't hold me to that).