Depends if it was caused by what needs repairing or if its pre existing.
Spray the mould with bleach to kill it off and wash with a damp cloth and dry out to stop it getting worse now that you have spotted it, I dont think it is your landlords reasponsibility to keep your home clean from mould as internal condensation is something we all have to deal with. I get it from time to time. Keeping the area clean will stop any further growth.
Keeping a reasonable flow of air inside a property will stop the mould growing as its a normal thing to happen if your windows and doors are closed all day and night.
Having windows open when its raining will also draw in moisture in.
My advice, keep a window open to circulate the air when your in and dont need the heating on only when its dry outside and ensure any wet surfaces dry off with a towel.
Hope this helps.