Clear/white poop.....may be an indication of bacterial infection inside the goldfish. But if this is a rare thing.....like you you would only see it once or twice a month or so you have nothing to worry about. Its normal. It just happens.
My goldfish sometimes poo's clear or white poop....and no worry here it just matters if the goldfish poops...that way everyday...or every other day. The last time mine poopes that way was a month ago....And he is doing just fine.
So how may times does this happen to your goldies? It all depends how often it happens. To determine how severe the condition is....If there even is one.