Am I supposed to know Machine Learning before I start my PhD in Machine Learning in Materials Science
Short Answer: No.
Long Answer: PhD in multidisciplinary topic expect that you know at least one discipline well. Then you get training in second discipline before applying the knowledge and ideas from both disciplines in order to fulfill PhD requirements.
My personal experience PhD. in biomedical informatics - I graduated in informatics, but nobody expects I have previous knowledge of medicine. (Of course - interest is expected or some courses is a plus). I did  some courses to gain the missing knowledge about medicine after (not full medical curicullum, but some physiology, patophysiology, basic anatomy, biochemistry ...). Thus, I think the same applies to your intention to pursue ML in Material Science - you can get courses of what you miss, supervisor may help you to direct what you need.