I have got following error in CM :
Bad : This host is in contact with the Cloudera Manager Server. This host is in contact with the Host Monitor. The host's Cloudera Manager Agent version (5.13.0) does not match the Host Monitor version (5.12.0).
My CM version is 5.13.0.
I dont know why Hostmonitor is showing old version.
As a first step, try to restart the "Cloudera Management Service"
According to the message you have upgraded Cloudera Manager Server and Agents, but Host Monitor is running on the old version.