You definitely say something. I had an assistant leave who had been with the district a while, I was in 2nd year. He did not leave on bad terms. He had kids messaging him quite a bit the year after he left. He screenshot everyone of them and sent them to me. Eventually, the kids stopped because we showed a united front and they learned to communicate with me.

It wouldn't be surprising if the kids initiate it. It happens, but he/she should be keeping you in the loop of it. The fact that he/she hasn't is a huge red flag. The fact that the kids are bringing it to you (if they are the ones who told you) is a bigger scarier red flag.