Considering the revamp, that FS had gone through, extensive reading of patches concerning the faction, this change has been put as an evidence of "We care for FS, FS is going through changes..."
There were a few places where it was referenced as Murkwater Weakness and rather than changing them all we just changed the name. This was always the original name of the rune. Just a house-cleaning thing.
But... the name is horrible. And Spellsurge is still: "Gloomwater" so you didn't clean house very well. Would have been better (from a consumer standpoint of course) if you fixed the internal naming scheme rather than rename something liked by the people, particularly since this is primarily for your own convenience rather than to prevent confusion or whatever on the part of the player (I never noticed any such inconsistencies).
When you casted the spell it has always said murkwater weakness actually. But yes, I forgot about Spellsurge.