Started solids - question

I started solids just under a week ago. So far he’s only had avocado, sweet potatoes, and liver so far. I only do a half oz of food at a time (homemade) and he doesn’t usually finish it all.

Anyway, he’s started having slimy smelly spit up and mucousy poo. It’s not after a specific item, it’s after any of them. No other signs of any allergies.

Is his tummy just not ready yet? Or is this normal when transitioning. He’s just over 6 months.

Tia