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.