Why do you think 30 minutes is required?

What is the humidity in the room?

Not required. Just making sure its not due to humidity.

Does your ceiling fan vent to the exterior (through the roof, etc). If not and it just vents into attic, that may be the reason why you have all the mold.

Yeah to the exterior. I check attic above. No mold there. Also the part just outside where we have the fan is alright. It is in side the walk in shower on ceiling.

Is there any chemical I could use to remove it completely and then may be see if it reoccurs ? Is there any special paint that is applied to the ceiling of walknin showers ?

How long has this been happening? Is the attic well-insulated? Wondering if you have some kind of strange condensation thing going on...

Does it wipe off or is it embedded into the ceiling? If it's just on the surface, something bleachy should work to clean it.

The mold is a result of humidity, what is the humidity in the room…Instead of running the fan for 30 minutes, maybe a longer run time is required.

The first step is a humidity reading.

You said you are running the fan for 30 minutes after a shower. Are you running the fan during the shower too? You should be running it just before, during and after a shower.

You can get a switch with a humidistat and it will run automatically until the humidity is low enough.

So humidity is only due to shower once a day. I run fan before and after. 30 mins just to be sure but after 30 mins there is not much of humidity. Also yeah there is some condesation on the ceiling of shower.

What do you think is causing the condensation ?

I think you need air flow, if your bathroom exhaust fan is not removing the humidity, add a fan, a floor based fan.

You may have more humidity than your exhaust fan can handle, purchase an auxillary fan.

Where are you located? What is your humidity?

" 30 mins just to be sure but after 30 mins there is not much of humidity. Also yeah there is some condesation on the ceiling of shower. "

But after 30 minutes is there still condensation on the ceiling? Have you actually checked the humidity level? There is nothing really to add or coat the surface with to make it resistant to the mold. My thinking is the fan is a) either not working properly, b) not strong enough, or c) not running long enough. How big is the bathroom? How many cfm's is the fan pulling?

Yes there is just insulation above the ceiling and then roof. Nothing else. So I think thats the condensation issue. No amount of fan would dry out the condensed water droplets sticking to the ceiling. However I'm sure it is not unusual to have shower just below attic right ? The roof guy had told me insulatio. Was good enough. Although attic is cold but thats how attic is right?

What's your climate or approximate location? It doesn't get cold enough where I live for insulation to be a factor. If you're in Los Angeles, for example, no need to worry about attic insulation having a part.

I'd spray it with Concrobium instead of bleach. Bleach will kill the surface mold but will do nothing to the mold that has penetrated the paper and gotten into the gypsum core of the ceiling. It'll simply grow back. Concrobium will penetrate into the gypsum, killing the mold roots as well.
After a few applications of Concrobium, prime the ceiling with something like Kilz then paint with a couple coats of a good semi-gloss or gloss paint.
Consider upgrading the bathroom ventilation to include a fan in the shower itself.
Add insulation in the attic above to minimize heat loss through the ceiling board and thus increase the temperature of the ceiling board, that'll help to minimize it's ability to be a condensing surface.

Thank you MongoCT I'll give these steps a try. Thank you all for taking time to reply.

I see a gap between cieling board and tile. Whats the best caulk to seal it ? I suspect thats what is causing mold. The steam is escaping to attic and condensing at roof and water deips down back to the top of ceiling.