Certainly is something, if not mold maybe oil? What is about this ceiling?

Might there be a leak up there?

That could be black mold. Don’t touch it. Have an expert mold tester come out ASAP.

Stachybotris

My ac unit is above on our roof and water runs down from it when the ac is on. Would that cause this? I've only had the heat on for now.

Is there a license needed to remove mold in California? Or can I hire my general contractor?

I don't know anything about CA regs, you should call your local building inspection office.

But just on the face of it, if this is all there is, it doesn't look like an extensive mold remediation job. It's two square feet of drywall. For one thing, you'd be hiring someone primarily to fix a roof leak, not to remediate your house for mold. I would hope there is a 'de minimus' provision in any regs for that. Unless your whole attic is full of mold.

In any case the mold is there because the drywall is wet because the roof is leaking.

You would want to search for a local remediation contractor that could repair any potential roof leaks and also properly remediate any mould.

For the moisture to be affecting your ceiling that much there is likely a lot of moisture in the attic.

You should have them review your attic to see the extent of the moisture damage... they should also be able from the damage patterns in the attic confirm the source of the moisture. Make sure they take care of the full extent of the damage and not just what is visible from below the ceiling. You don't want to leave anything festering away and possibly affecting your health after you have gotten it all corrected.

If you want an unbiased diagnosis you could find a local "building envelope consultant" that could do an inspection to determine the cause and recommend a proper repair.