Hi,
Could you look at security flow sessions for this traffic:
show security flow session source-prefix x.x.x.x destination-prefix y.y.y.y protocol icmp show security flow session nat brief show security nat source summary
You can follow the steps in this guide:
https://kb.juniper.net/InfoCenter/index?page=content&id=KB21719&actp=search
Cheers,
Ashvin