Good guess Pat (though I'm sure you weren't guessing), I found this on the Convatec website-they supply ostomy stuff:
Urine crystal formation is a condition most commonly caused by alkaline urine. Its appearance is that of white, gritty, crystalline deposits on the stoma, peristomal skin, and/or lining of the pouch. While treating the condition that causes alkaline encrustation, urine crystals can be dissolved by using a dilute solution of vinegar and water to wash the stoma and peristomal skin.
Alkaline urine is a common cause of urine crystal formation. Reducing alkalinity and acidifying the urine can be achieved by increasing your fluid intake to two or three quarts of fluid per day. A medical professional may also recommend ingesting two grams of ascorbic acid (Vitamin C) daily, in order to keep the urine acidic.
When crystal formation is caused by uric acid in the urine, a condition that rarely occurs in the population, the urine needs to be diluted by increasing fluid intake and alkalizing the urine. Please see your physician for specific treatment.