leaving the .net off is pretty standard in the industry and making special note that it is Mono C# using Mono MCS is standard. It's like when someone says they have an SQL server you don't ask them if they are using an MS SQL server or a MySQL server... MS SQL Server is assumed unless otherwise noted. Or when you say C++ you have to specify it's VC++ to indicate your using microsofts compiler or it'll be assumed to be gcc.
Those particular assumptions may be less common/popular than you seem to think; they're certainly not universal to the profession as a whole.
Taking a couple of specific examples, it would actually surprise me if a professional programmer assumed my C++ code was compiled with gcc simply because I hadn't mentioned Visual Studio; that's a completely alien assumption to me that I've never encountered. Depending on what sort of programming is being done, there are probably plenty of programmers who wouldn't assume gcc OR visual studio unless they were explicitly told so.
If someone wants me to work with an SQL server, asking whether they mean MS SQL, MySQL, or some other variant would be one of my first questions; and at least in my own experience, the answer often isn't MS.
unity doesn't Actually use C#... it uses Mono-c# which tries to be as identical to c# as possible
C# is a standardised language, and you're using the same language either way. While it may be noteworthy that Unity uses Mono rather than the .NET runtime, in my own personal experience most programmers would consider it to be incorrect to suggest that you're actually using a different language -- it's just a different implementation.
To reiterate the above, your own assumptions may be less universally accepted than you seem to think.