Generally speaking, sunroofs and moonroofs leak because the drain hoses
are clogged. It varies by manufacture, but often there is a drain in
each corner with hoses that run down the A and C pillars, the seal
around the panel was never meant to be water tight in the first place.
Not sure if this is the case with yours, but that's where I'd look first.

I agree with James - look for plugged drains. Cleaning them may be a job for
professionals.It would be a hopeless thing to try to seal the moonroof;
you'll never get it to hold for long.

Mike