Redirecting the output is important to allow the ssh process to exit:
pssh/bin/pssh -h ple.txt -O StrictHostKeyChecking=no -i -l irisaple_pacemaker2 "cd client; nohup ./client > foo.out 2> foo.err < /dev/null &"
pssh/bin/pslurp -h ple.txt -O StrictHostKeyChecking=no -l irisaple_pacemaker2 -r -L data /home/irisaple_pacemaker2/client/log ./


pgrep instead of pidof