Cystic mass as you mention could be from some other structure inside abdomen or abdominal wall besides arising from UB itself. So as a first step, your surgeon has to ascertain whether it is from abdominal wall or not(common) secondly if it is not from wall then from which area likely? this can be done on CT-scan. Sometimes it can be from intestinal wall also.
In brief you must be investigated for the site of origin.They are generally benign and produce little symptoms.After knowing the origin it can be resected.
is benign as your tests show, but since residual urine is more it may need to be removed