Well do you normally see anything comming out of the top?
If you don't know the mouth is on the bottom and has quite a unique engineering design, the anus and --- organs are located at the top. could be that your urchin was simply expelling waist. As to the color, depends on what it eats. If it were a milky substance then it would have been sperm and would mean that your corals would be feasting on it.
Thomas