This is the query I use to see spot healing done during Star Augur on Icy Ejection targets:
(resources.hitpoints - effectivehealing) < 2000000 and in range from type = "applydebuff" and ability.name = "icy ejection" to type = "removedebuff" and ability.name = "icy ejection" group by target end
If you just want to see damage done to targets below a certain HP treshhold, you simply use:
(resources.hitpoints - effectivehealing) < 2000000
You have to add the "- effective healing" since otherwise it will only show healing to targets that were below the treshhold after the heal was cast.
This video is a pretty good guide for using queries/expressions from the creator of warcraftlogs. At 29:30 he talks about how to use expressions to look at healing:
https://youtu.be/mHyS3WzVmgQ?t=29m30s