It could be from some kind of medicines they had going on in the tanks for them, and now since you've brought him home and put him in some fresh clean water it's all fading away and disappearing. I hope that's what it is--I hope he's ok.

Is he swimming/eating ok on his own? I think I have seen this before a few years ago on a few bettas at Wal~Mart. No offense to Wal~Mart but they don't seem to care for their fish very properly and lots of them die and have ick and stuff wrong with them all the time. Where did you get your betta??

Give it til tomorrow maybe and if it's not any better by then it probably won't get better unless you get some medicine. I'm still thinking maybe ick drops might help???