Feral cat meows to get my cat's attention

[Shuashushs]

Hi! This is my first time posting on my site so apologies if I post this thread incorrectly.
I live in a neighborhood with a colony of feral cats; I don't take care of them since my house is located at the end of a driveway (like a cul-de-sac but even narrower) where there are a lot of dogs and cars around. A woman way outside of the driveway feeds them, which is where they typically wander, but once in a while, they like to pass through my yard briefly. Currently, I have an 8-month-old cat that I've been caring for since October; she is spayed, has all her necessary vaccinations and such, and is an indoor cat.
Recently, a particular feral cat has been walking up to my doors (both front and back) and meowing, to which my cat will hurry over to the door and meow with them. My cat doesn't seem agitated; she just sits and meows back. The cat runs away even when I'm a few feet away from the door (as long as I'm in its vision I believe) so I believe it is feral; because it runs so quickly the only thing I can see about it is its grey fur. It likes to come throughout the afternoon and probably meows for my cat maybe 5-7 times per day separately. This has been happening for the third day straight.
Does anyone know why this might happen? I've read that cats do not even meow at each other so I'm quite confused.

 

[Caspers Human]

It sounds like your kitty has a cat buddy!

Casper has a cat buddy, outdoors, too. The only difference is that Casper's cat buddy has her own humans.
Her name is "Louie" and she lives two doors down. Her humans let her outside and she comes up on our back patio to visit Casper.

Casper will often sit near the patio door and wait for us to slide it open so he can visit with Louie. It's kind of strange how, sometimes, he seems to know when Louie is out and about.

Louie is a very nice cat. Sweet and very friendly.

There is another cat that isn't very nice. It's a calico and she's rather bitchy. There was one night, I was going to bed, when I heard noise outside on the patio and I went to investigate. The calico was there. I didn't want to encourage her so I locked the door and pulled the curtains. That darned cat tried to take a swat at me THROUGH the glass! I was even a couple-three feet away from her too, so you can't say that she thought I was making a move toward her. I don't know where this cat lives or who her humans are but she isn't welcome on my porch. I scare her away any time I see her. I haven't seen her in a while, now. Either she's gotten the idea that she's not wanted or something else has curtailed her visits.

On the other hand, Louie is welcome to come visit Casper any time she wants.
Louie comes up to the screen. Casper and Louie will sniff each other through the screen. Louie will often sit on top of our patio table, in view of the patio door and Casper will lay on the floor, near the door and the two of them will just chill.

Louie doesn't come every day but a few days a week, pretty regularly.

When she does come and we see her before Casper notices, we'll open the patio door and call Casper, "Casper! Your girlfriend is here!"

Casper always comes a-runnin'!