String like substance on my newly bought betta's tail

[xenogade]

This betta was bought on sunday and put him in a tank with age RO water. I have also put in ketapang leaves. When i came back, found out some string like substance hanging from his tail. However when he swim around, the string started to fall off. Could it be that he is shedding his slime coat? i also didnt notice any signs of the fin rot. any seniors can help? Below is the picture of the fish

Nothing too worry. Let the fish keep a rest and don't disturb it.
Very nice red