TMSL Alter Column Description

The ALTER command only works with the objects listed in the documentation listed here https://docs.microsoft.com/en-us/analysis-services/tmsl/alter-command-tmsl?view=asallproducts-allversions 

(Database, DataSource, Table, Partition and Role)

If you want to alter the description on a column you would actually have to specify the entire columns collection and all the properties for all the columns.

For this reason I would not use TMSL to alter a column description. I would either use something like Tabular Editor or if you want to script this you could use PowerShell or C# with the TOM library (which is part of the AMO download here https://docs.microsoft.com/en-us/analysis-services/client-libraries?view=asallproducts-allversions ) and a version of this is usually on your machine if you have the SQL client tools installed.