If it is simply white, it should be fine. If it appears to have large mineral chunks, like the consistency of sand, that could indicate a problem. Most likely, this is just normal.
To add to what tonyshuman has posted, your bunny is excreting excess calcium. The whiteness that you are seeing on the poo is from the pee. I would not be too concerned unless the pee is gritty and looks like sand. How is your bunny's water consumption?
Here is another link to information on bladder stones/sludge in rabbits:
Besides water, how old is your bunny? With excess bladder sludge, your bunny is getting to much calcium. Babies need calcium rich diet--brocoli, alfalfa, etc. Older rabbits that get too much show with the urine that they are getting too much. Can lead to stones in the bladder leading to infection and surgery or death.
my bunny is two months about, she eats about 2 bowls of food a day. a big bowl of alfafa and sometimes spinch and carrots, and she drinks a huge bowl of water a day.
She sounds healthy to me. As long as it looks like the pic I posted and doesn't appear to have sandy crystals in it, it's totally in the range of normal.