From the Message Guide, a TN5157E message means "The number of running monitors specified by the MAXACT value in TUNSSPxx has been reached. No new running monitors will be started until current ones end." The monitor measurement is cancelled. The recommended action is to review and modify the MAXACT value in TUNSSPxx.
If entering the "P" command is not working on the status panel, and the cancelled monitors are only purged after CA MAT is recycled, then check to see if RO51759 is applied. It fixes a problem with control blocks being left behind when monitors are attempted, but fail.
This APAR fixes a problem specific to a monitor failing because the targeted job is not found. The monitor is left in a waiting status. However, an attempt to stop the monitor puts it into a stopped status and it still doesn't go away. There is a control block that is built when we are initializing the monitoring environment that is getting left behind if a monitoring attempt fails looking for the targeted job. The same code and same control blocks are used for all monitoring environments built in the server, so whether the monitor is unable to be start because the target job is already gone, or because you have reached maximum active monitors probably doesn't matter.
If after applying RO51759 the issue persists, please open a new issue through CA Support Online. Please provide CA Support with the complete MATUNER STC LOG, and your maintenance level (last PTF applied, or run a SMP/E LISTMODS on your CSI and provide the complete output).