Yes because you are using a var. A var is not a VECTOR. A VECTOR has .x, .y, .z but a var can only have a floating point value.
STRING* data_str = str_create("#100");
STRING* temp_str = str_create("#100");
str_for_num(data_str,mouse_pos.x);
str_for_num(temp_str,mouse_pos.y);
str_cat(data_str," ");
str_cat(data_str,temp_str);
//Use str_remove() when you don't need the strings anymore
This generates the following string: [value of mouse_pos.x][space][value of mouse_pos.y]
You should be able to read out the values after you sent them
