Pretty natural for it to grow, but will be unhealthy if there's significant amounts, and can get out of hand if left alone. Bleach + paper towel works well, though the mold can leave dark discoloration in some sealants, so don't worry if it doesn't return a perfect white (leaving bleach to soak can also help).

Dryness prevents it, so make sure those areas are ventilated and not too damp, but it is generally inevitable in places like a shower and just needs cleaned every now and then.

Prevent it by dealing with the source of the moisture, which is likely the shower if it's only in the bathroom. If you have a bathroom exhaust fan, use that during and after showers. Opening a window can help if you don't have one.

When my vent fan died last winter, I used a small dehumidifier because the moisture got really bad. Dehumidifiers take a fair bit of energy, but that energy is converted to heat, so it's basically the same as using electric heat.