Mold is a real threat when moisture is allowed to remain in the ceiling and the space above it. It can take a little as a few days for mold to begin growing inside a ceiling, so acting quickly to get everything completely dry is the best course. In addition to being a health hazard, mold can eventually cause structural damage to the home if left to grow and thrive.
Minor ceiling cracks are not uncommon and may simply be the result of the home settling. Even older homes can continue to settle for decades. If the cracks are minor, they can be repaired with relative ease. The technique used will depend on the ceiling material. Old plaster ceilings will often sag and crack over time, and can be patched with drywall instead of new plaster. Popcorn ceiling cracks can be covered with aerosol sprays made specifically for this use. Major cracking can be due to seismic activity or structural issues and should be assessed by a professional before repair or replacement.
Paying attention to the ceiling can really pay off by allowing the discovery of small problems before they can become big ones - avoiding serious and potentially costly problems down the road.