Help please!! Yellow and black poop in 1 mo baby stool

Hello guys, sorry if this is gross but I am sharing below a picture of the stools of my 1 month old prince. It's been about 4 days it went from pure beautiful mustard color to those streaks of black in yellow. Is this normal? He is 80% formula fed, and the rest comes from my low supply breast. Today it has the texture of smooth peanut butter but the past 3 days it's been rather hard and pebbly. His stool frequency is once to twice every day.

I've heard there was an app where you could share pictures of your baby poop to check if it is alright, has anyone heard of it ?