Stop macro from chatting?

[Umo]

<lasttarget> displays the name of your last target, that's why you're having the mob name displayed on chat.
As long as you have /ac Cover <2> you're not changing target (I mean, you're character will cast the Cover skill on <2> without actually targetting that character), so just delete that last line and the macro should work perfectly fine.

On a side note, consider to add a party chat line so the covered character may be aware incoming damage is redirected to you. Some unexperienced healers panic and tend to heal themselves ad nauseam when attacked, even though they are covered by the tank.
Something like /p {Cover} on <2> {Just used.}! should work on most cases.