I think your problem is that you're feeding way too many veggies. It can cause unevenly colored, soft poop. Pigs can also quickly come to prefer veggies over hay, and that can cause major, expensive tooth problems.
I can certainly attest to this! We had exactly the same problem with my Chester and it led to teeth planing and anesthesia every 4-6 weeks. The boy's little body finally just couldn't take any more trauma.......and Chester, the miracle pig, finally ran out of miracles.