There's a lot of fuzzy terminology usage, but in general "Multi Cloud" refers to multiple Cloud vendors (e.g. IBM and AWS) whereas Hybrid Cloud refers to a combination of on-premises and in Cloud. In reality, almost everyone will end up being Hybrid Multi Cloud, so it's kind of a meaningless distinction.
Furthermore, if you're using a Cloud service through REST, you may not really know where the Service actually resides nor what Services it in turn calls. Typically both the terms "Multi" and "Hybrid" refer to an individual or organizations point of view and the computer centers that they are directly connected with.
Regards,
Glen Brumbaugh