With mould that large, Im not so sure - if its thick, it means its been sitting there for sometime and chances are has eatn through the coating.
If its thin, seethrough and you seemed to have gotten it early, I think the glass would be fine - and the actuall cost to image quality would be minimal.
Get it serviced asap and Im sure you will be fine

_________________
1) Olympus OM1 [Zuiko Auto-S 50mm f/1.8]
2) Pentax MZ-60 [Sigma 28-90 & 100-300]
3) Canon 7D [EF-S 15-85 & 70-200mm f/4 IS]
Leo's Flickr Page