just keep an eye on him/her. If he/she remains restless for a few days something may be wrong. Stressed axolotls can form white skin patches which can drop off when stressed, it could be this. Another reason for restlessness is they are trying to breed. That white thing may have been a spermatiphore (salamander sperm) is there any other axies in the tank which could be getting him in the mood?
its hard to tell what it is if we cant see it. Ceep an eye out, hope he/she is ok.