It is just bacteria growing on the food and breaking down.

 

Hi! Welcome!!
You need to clean up that uneaten food immediately. Buy a new large-sized turkey baster, and keep it for tank cleaning only.

 

yup, it's bacteria! I'm pretty sure you're over-feeding too. Like Mbpoppy already told you, uneaten food needs to be cleaned up immediately. If it's left to sit it will cause ammonia spikes which can easily make your fish ill or even kill it.

 

Is there a reason this is happening now? We never had this issue with the previous fish with uneaten food.

 

Did you make up the tank entirely new? Or did you add anything new to the tank recently (beside the fish)?

 

We used the same tank after I cleaned everything. We did buy two small little sculptures (mushroom and turtle) that we did not have prior.

 

Maybe this fish isn't eating as much as he should be, or he's young and needs smaller meals more frequently. Every fish is different, just like people, or cats