class() will show you the data type for its argument.
adaptfilt.lms returns a "FIR LMS object" according to the documentation. The datatype involved will not be one of the fundamental arithmetic data types such as "double", and it will not be a cell array or a struct: it will be an Object Oriented Programming (OOP) object.
I do not know if you are permitted to pass such objects to a MATLAB Function Block (that is, this is outside my experience.)
