"Stay?"

Phalanx repeats it the first time out of surprise, sitting up a little (only after Robin seems to be done doting) and turning to look at him. They both might be attuned to subtle changes in atmosphere and energy that mortals could only dream of detecting, but looking at a face still helps Phalanx figure out what someone means with their words.

Then, the second time that Phalanx repeats the word, it's phrased as an answer.

"Stay."