Water Damage on Ceiling of Shower
To fix the damage, pry any loose material free, patch with compound, sand, and prime/paint with a paint designed for wet areas (preferably mold/mildew-resistant too).

To prevent this from happening again, you'll need to improve ventilation in the area. It's a little hard to tell from these photos, but this looks like a corner within your shower where warm moist air is likely to pool. A drywall or plaster ceiling that is constantly exposed to moisture is going to have these problems again. Consider some of the following options:


Ensure an exhaust fan is used when the shower is run, and for 30min afterwards to clear moisture from the bathroom.
Add an exhaust fan, or reposition the existing one, to better ventilate this area (Note: you probably can't safely/code-compliantly put it in the shower area)
Lower the glass divider, in order to provide an air space along the ceiling so that moisture can disperse more easily.
Have less steamy showers (your shower head type and position, as well as the typical water temperature, can affect how much steam is generated)
Make the ceiling more water resistant - better paint and caulking any cracks may be adequate, but you could go as far as adding a waterproofing membrane and tiling it.