Looks like diarrhea, is he showing any other symptoms of sickness like lethargy, fluffiness, inactivity, not eating/drinking or quietude?
I would take him to an Avian vet for a check up!
Hope your little one is ok!
and Princess Mallorn!
Thank you to Deb for her wonderful Faery magic
Moults can definitely have an effect on poop, they can have watery droppings due to the normal stress caused by the moult. After eating leafy greens or fruit they can also have a couple of watery droppings that soon go back to normal consistency.
You should only be concerned if the this pattern is consistent and you notice a decrease on your budgie's activity levels and exercise.
Keep an eye on your budgie and depending on how he progresses then you'll decide if it's best to take him to the vet for a consultation or not.
EDIT: Oops, on my third line I meant a decrease in budgie's activity levels and appetite.
RIP sweet Tito (Summer 2008 - January 17th 2013).
You are missed and never will be forgotten.