Columnstore tables (in MemSQL 4.1 and above) have a hidden rowstore table that buffers small inserts into the columnstore table in memory. When enough rows have accumulated in memory the rows are removed from memory and flushed to on-disk columnar segment files. The memory use for this hidden rowstore table will show up the same way as a regular row store table.
Columnstore tables also store metadata about the columnar segment files stored on disk in memory. Each file has a row stored in memory with some information about this file (maximum and minimum value of the column in this file, bitmap of rows that are deleted, etc.). The memory use for columnstore metadata is currently not broken out into a seperate component, but shows up as rowstore memory use along with all other row store tables. The memory use for columnstore metadata should be small unless the columns are storing a large number of bytes (for example, if the maximum or minimum values are large).