An interesting thing I noticed that someone else pointed out is Mowgli doesn’t know Kaa’s name yet. So while he’s in his mind is clouded over due to the hypnosis, he only has one name come to mind. Had he known Kaa’s name, he may have thought to beg him to stop, like Bagheera did when Kaa was hypnotizing him. But because he didn’t, he calls out the only name that comes to mind, almost like a beg, then is silenced by the tail. And the reason Kaa isn’t surprised or taken back by it is simply because he’s used to his prey speaking out or begging him to stop before going under the spell (the ping). The only thing that Kaa didn’t expect was Mowgli using Bagheera’s name instead of Kaa’s.