Foster cat

I am fostering a cat for the first time ever. I have a resident female cat that is about 8 yrs old. The foster is a male, 13 yrs old. I have had him about a month and he has his own room that he lives with play things etc. He is well cared for. I have let him start coming out in the home when I am on the living room so I can keep an eye on him. My resident cat is cautious and sometimes growls at him but no fights or anything. The question I have is why does my foster cat meow at my resident cat or just in the living room? Is he trying to say hello, "I'm friendly" ? It's not a growl or a stay away meow. If he sees my resident cat watching him he will meow several times at her but won't approach. Or he will sit in the dining room and meow over and over while he is exploring. Any ideas? I am not a car communicator and am not sure if he is trying to be friendly or if this is something else. It's hard to explain without sounding like I'm asking why a cat would meow at all. 😆 It just sounds like a quick repetative meow at her like he is saying hello and making eye contact with her, but neither of them approach each other. He is still super skittish and won't even let me touch him yet. I'm being patient and giving time. TL/DR: why is my foster cat meowing at my resident cat? What does it mean?