Here are a few rules of thumb to recognise mould:
Mould is always dry and furry. Mould can not develop under the surface because it needs oxygen.
If you see reddish or greenish-grey spots or a coloured coating on your yoghurt, this is a sign of mould or unwanted microorganisms. The main reason for mould growth is usually poor hygiene. Perhaps the fermentation vessel or the utensils used were not cleaned intensively enough. Take special care that if you only eat part of the yoghurt, you remove it with a clean spoon.
Mould forms spores and if you find mould on your yoghurt, as painful as it is, you must always dispose of the entire contents of the container and clean the container in the dishwasher.