Thank you for contacting Toronto Master Gardeners.
From your picture and description I am unable to give you much information.
The fluffy pieces on your leaf look to be seeds stuck to the leaf.
If you can send a picture of the plant showing what part has been attacked and damaged tissue we may be able to assess this further. It will also help to know what type of plant it is and what symptoms you have observed. The location of the plant ( near a window? how much light?) and watering habits will also help with figuring out the problem.