[in the UK] I've just bought some 'Ronseal 3 in 1 mould killer' spray from B&Q -- also available on Amazon -- which has worked well on my polystyrene ceiling tiles in the bathroom. These were quite mouldy in places (due to problems with my roof).
The spray worked really well (and the spray mechanism was very effective - giving a good-coverage fine mist spray rather than dribbling out (onto me) or producing a 'squirt' covering only a small area).
I didn't try to remove all the mould-discolouration from the tiles as I will be painting over them (which is an option if you don't wish to remove the tiles from the ceiling as suggested by some. I'd love to get rid of my crappy ceiling tiles but I know I'd regret it given the resulting mess and cost to make it good).