My baby sometimes does this, seems like spit and bits of curdled milk when I’ve seen it!
I would say it’s possibly a bit of reflux or certainly sounds like it from what you’ve said. If it isn’t causing him any distress then I wouldn’t worry x
Mine does this often when he’s just waking up. I’ve always figured it’s a bit like when we wake up with dry mouth? Be interested to see if anyone has an answer!