SQL Server is a registered trademark of Microsoft at least in the US and I’m sure other jurisdictions. Using SQL Server to refer to Microsoft’s relational database product is perfectly reasonable and pretty much every will know what you mean. The generic term seems to be RDMS or some variation on that.
Referring to a RDBMS as a "SQL Server" would probably confuse people. Besides the fact that it's already the name of a specific product, the overall genre of software doesn't even serve SQL.
