You, er, can also remove the /castsequence and reset portions. ;) If you only have one spell in the sequence, they do nothing. Also, "help" implies "exists"; you might want to include "nodead" in the first two, though. This should have the same effect:

#showtooltip
/stopcasting
/cast [@mouseover,help,nodead][help,nodead][@targettarget, help][@player] Greater Heal

That is, cast GH on your mouseover target if that target is friendly and not dead; otherwise, your current target, if that target is friendly and not dead; otherwise your unfriendly target's target, if that target is friendly; otherwise the player.

@Chris Anthony:

Yeah, I'm sure they could be cut down some. I modified them from older macros I found somewhere or 'nother, and I figured, if it ain't broke, don't fix it. :D They work as-is, and they don't break the character limit, so I never bothered to trim them down.

Understood. :) I'm an efficiency fiend because my macros tend to run quite long, so I'm always on the lookout for ways to slim down. Here's an example (uncontrived; I raid with this macro):

#showtooltip [mod:ctrl]Power Infusion;Pain Suppression
/cast [mod:alt,mod:ctrl,@player][mod:ctrl,@mouseover,help][mod:ctrl]Power Infusion;[mod:alt,@player][@mouseover,help][ ]Pain Suppression

@Chris:

You should be able to trim the bit after #showtooltip, it should show the correct icon and tooltip based on the modifier logic in the rest of the macro, so that should save you a couple characters (and a headache if you change the macro modifier logic but not the tooltip line).

I wish you could nest conditions, so that we could write your macro as something like this:
#showtooltip
/cast [mod:alt,@player][@mouseover,help][]{[mod:ctrl]Power Infusion;Pain Suppression}

Alas, I don't think there's a way to do that, so we gotta write it out long-hand, which is error-prone and annoying, particularly for complicated macros.

@agnoster, that's certainly generally true. I recall there being a couple funny edge cases where #showtooltip doesn't do what you expect it to unless you include the spell names, but now I can't remember what they are, and honestly it's probably safe to remove the spell names altogether. :)