I'm guessing it could be one of two things: constipation or eggs. Do you know the sex of your goldfish? How long has it been going on? Is it a new fish?

If the poop is white, it is probably constipation. Don't feed for three days, then feed only blanched peas for three more days. It should be clear by then if that is the problem.

If it is clear, the bubbles are most likely eggs. If you are sure that your goldfish is male...then it may be something else

How young can the fancy goldfish start having eggs/babies?