There are options specific to back up from a replication slave.
This option is useful when backing up a replication slave server. It prints the binary log position and name of the master server. It also writes this information to the xtrabackup_slave_info file as a CHANGE MASTER statement.
This is useful for setting up a new slave for this master can be set up by starting a slave server on this backup and issuing the statement saved in the xtrabackup_slave_info file. More details of this procedure can be found in How to setup a slave for replication in 6 simple steps with Percona XtraBackup.
In order to assure a consistent replication state, this option stops the slave SQL thread and wait to start backing up until Slave_open_temp_tables in SHOW STATUS is zero. If there are no open temporary tables, the backup will take place, otherwise the SQL thread will be started and stopped until there are no open temporary tables. The backup will fail if Slave_open_temp_tables does not become zero after --safe-slave-backup-timeout seconds (defaults to 300 seconds). The slave SQL thread will be restarted when the backup finishes.
Using this option is always recommended when taking backups from a slave server.
Make sure your slave is a true replica of the master before using it as a source for backup. A good tool to validate a slave is pt-table-checksum.