ZMMK invests in high quality money market instruments issued by governments and corporations in Canada, such as treasury bills, bankers' acceptances, and commercial paper. Its top holdings include commercial paper and bankers' acceptances from Central 1, Bay Street Funding Trust, National Bank of Canada, TD, etc. These are mostly short-term debt instruments issued by large Canadian corporations, paying a high yield.
On the other hand, CASH invests in high interest deposit accounts with Canadian banks. Its holdings are much more concentrated, with the fund holding funds in cash accounts with National Bank, CIBC, and Scotiabank.
The main difference is ZMMK invests in short-term debt instruments and CASH places money in high interest savings accounts. We would consider ZMMK to be 'safe', however, as these are debt instruments as opposed to purely 'cash accounts', we would consider it to be slightly more risky than CASH.