It might be poop, he could have been constipated and now is "cleansing". I know it's hard and your probably very worried right now the best thing you can do for your baby is to fill out the info in the sticky. I know it may seem redundant but it is very important and helps to narrow down possibilities. I pasted the questions to make it easier just fill them out and we're one step closer to helping the little guy out
What size is your tank?
What temperature is your tank?
Does your tank have a filter?
Does your tank have an air stone or other type of aeration?
Is your tank heated?
What tank mates does your betta fish live with?
What type of food do you feed your betta fish?
How often do you feed your betta fish?
How often do you perform a water change?
What percentage of the water do you change when you perform a water change?
What type of additives do you add to the water when you perform a water change?
Have you tested your water? If so, what are the following parameters?
Symptoms and Treatment
How has your betta fish's appearance changed?
How has your betta fish's behavior changed?
When did you start noticing the symptoms?
Have you started treating your fish? If so, how?
Does your fish have any history of being ill?
How old is your fish (approximately)?