A.B. Channing "Red Rover" PDF

The Redding will take you if it knows your name. This is the rule that has kept Meg alive – or rather, awake – since the night the world went to hell. Nobody knows where the red substance came from, but it acts like water; it's in the water and falls as rain every night. Those captured by the Redding fall like marionettes with their strings cut. They are not dead, but asleep, and can be awakened if their name is spoken again. However, they will only try to kill you.

Meg and the other survivors she has befriended think they have found a stable equilibrium six weeks after the catastrophe began. Nicknames have become real names, and Meg has become better at identifying safe houses. Her crush discovers that Morse code, like writing, is a safe way to say a Sleeper's name. It's the night after he learns this that Meg wakes up just as the nightly rain is starting.

As it turns out, raindrops can make dots and dashes. The Redding can talk.

And it's telling her to run.