execution plan question
It means that the parameter should be defined to match the data type of the underlying column - currently the parameter is nvarchar(4000) but it should be varchar (looks like 255 but you should check the table).

Whether this actually makes your query slow or leads to a bad plan is tougher to determine. But making those two definitions match certainly isn't going to make it worse.

If you can't change the parameter data type, either get a better ORM or change the column as a last resort.