Update: According to this comment as long as you have a balance of at least $1,000 your account will be in good standing and won’t be closed.
Morgan Stanley is closing the Cash Plus accounts of some customers. It seems to be affecting customers that were paying the $55 monthly fee, rather than having $25,000 parked in the account. This was worth it to avoid paying the annual fee on the American Express Platinum Morgan Stanley card (discussed here).
There is some further discussion of this in this FT thread and also our thread here.