He didn't really want to talk about Medusa, especially not in front of a boy whom he had with another woman. Some old tales were better left buried, and sometimes he would admit that their on-again, off-again relationship of strange sorts carried on against his better judgement. She paid the price for the lesson he was supposed to learn, and perhaps that was why he never quite learned from it.
It was, however, no big secret, nor did Poseidon usually bother to try and hide anything that seemed to be worth hiding. Trying to keep secrets when everyone's lives were documented on paper was one way to start the infighting all over again.
"I was with Medusa when she came here a while ago. She's been taken, and I have to get her back." It was the right thing to do. It was the only thing he could do.