Honestly, she might not have anything against him specifically. Asterion is likely a form of scapegoat to an issue among the gods. Maybe she just has a problem with Poseidon and she targeted Asterion to punish Poseidon. Or it's an elaborate plan to keep an eternal worshipper of the olympus, given that gods might require followers to have power and overall health, like some fucked up form of a retirement plan she devised.