Sometimes yeast flocculating on the top of your starter looks like mould. It is actually rather beautiful, and in this instance you will need to trust your sense of smell. If it smells foul then discard it.
If you see black or red mould then you must also discard your starter.
Occasionally you can rescue a stater that has mould on it. You have to be very careful, get rid of all of the top of the starter. I recommend doing this outside as mould spores are a threat to your starter and can linger in the air for a few days waiting for somewhere to land.
If you find a creamy uncontaminated section at the bottom of the pot then you can double refresh with rye flour at 28C and see if it shows signs of life. If not, start again.
