How can I center this text just on mobile?

 You need to use a media query in the Custom CSS to target mobile (using screen max-width), and then target the text block with the block ID, and then set the text alignment to center:

@media screen and (max-width:749px) {
  #block-yui_3_17_2_1_1660700494123_911821 {
    text-align:center !important;
  }
}

Let me know if that works for you!