Hi ZhanZhan, I can see a clear picture that will show what type of plant you show, the importance of knowing the plant type help somewhat to identify the mould or fungus that has attacked the plant, there are certain plants that will get one type of fungus / mould but, others wont ever get it, there are others that every year suffer Grey Mould (hollyhocks) yet the different plants next to these are trouble free.
Healthy, well fed or cared for plants can normally fight off an attack from aphids, diseases or a little stress so a bit more info re the kind of plant that is in trouble would be better help and the right info for a cure is far better than guess work from all and sundry.
Please get back soonest with the type of plant you are concerned about before it dies completely.
Good luck and best regards. WeeNel.