Mouseover healing when not the master is tricky with keyclone, so I recommend either setting up healing macro hotkeys like exampled below, or running the healer as master.

The targeting parts need to be in brackets too, I'm not sure of the exact syntax for having the mod part in there as well.

In any case, the easiest way to do it is to have different buttons for healing each character.

(and you can use @ instead of target=)
For example:

/cast [@self] Healingspell

and on a different button:
/cast [@party1] Healingspell

Though I find it easier to make the targeting by name so that it'll still work if you end up in different groups in a battleground for example.