It doesn't make your next heal insta cast - that is what Nature's Swiftness does.
Unleash Elements with Earthliving Weapon imbued is a small instant heal on your target, which gives you a buff which increases the power of your next direct heal by 30%. It's quite potent when combined with GHW or Chain Heal. It's not great when used with healing Wave as if you are using HW then the damage probably isn't that great anyway, and if you're using healing Surge the damage is probably coming so quickly anyway that using a global on UE might end up getting someone killed.
Telluric currents is very weak really - people from Paragon use it because they are on Sinestra progress and that has a big mana less burn phase in the middle where you can regen silly amounts of mana with it. verall the movement speed buff from Ancestral Swiftness is far more potent.
Here is the build I use on Slash:
Focused Insight is another one of those weird talents which is more useful in PvE than PvP.