Fixing slow water damage at top of wall
Are you a condo owner or are you renting an apartment?  If you're renting, take good, clear photographs showing the location, extent, and appearance of any damage before you report it and keep dated records of all communications and changes in condition. Before doing any demolition to look into the walls or ceiling contact your landlord/HOA/POA first.

When I lived in an apartment, the landlord had some changes made to the deck above and beside my unit. It caused rainwater to run into the wall and ceiling of my unit, causing major mold and structural issues.  When I moved out he tried to say I damaged the unit and bill me for the repairs but I had dated copies of photographs and letters where I notified him in writing of the leaks, resulting damage, and requested repairs, showing it went on for over a year with no response from him.  I got my whole deposit back.

Can you post a picture to help us understand the extent and location of the damage?