You can’t remove <span class="acf-required">*</span>. You need ot make the field not required or use custom CSS to hide it.
Changing error messages requires using JS. see https://www.advancedcustomfields.com/resources/adding-custom-javascript-fields/ and https://www.advancedcustomfields.com/resources/javascript-api/ I don’t have any examples. You might find something here or on similar topics https://support.advancedcustomfields.com/forums/topic/acf_form-with-custom-validation-message/
