David Harewood is still a regular I believe, J'onn just doesn't work at the DEO anymore.
That's correct. J'onn explicitly said he wasn't leaving his friends, just changing his career. He's "among the people" now, whatever that means. If the storyline is focusing on anti-alien extremists again, then giving J'onn a prominent role in National City's nonhuman community could tie into that.
Wild guess -- maybe J'onn becomes the new owner or bartender of the alien bar. That's an established set, so it's economical to keep using it, and it's a regular gathering place for the nonhumans of National City.