Whenever I get a stack of Maelstrom Weapon the button lights up. This is very annoying since optimally you use Maelstrom at 5 stacks. Is it possible to change this so it lights up at 5 stacks? For the life of me I can't figure out how to do it manually.

It's a current known issue, for now if you want you can edit the \Interface\AddOns\LibPlayerSpells-1.0\data\Shaman.lua file line 157 and change:

[ 53817] = {403, 421, 8004, 73920, }, -- Maelstrom Weapon => Lightning, Chain Lighting, Healing Surge, Healing Rain (Enhancement)

into:

-- [ 53817] = {403, 421, 8004, 73920, }, -- Maelstrom Weapon => Lightning, Chain Lighting, Healing Surge, Healing Rain (Enhancement)

That should disable the aura stack counting but the shine will still happen at 5.

Maybe should this be added as a ABA rules, only displaying the stack count and an hint at 5 stacks ?

You can also disable the rules using the configuration panel. See "setting up" there : https://github.com/Adirelle/AdiButtonAuras/blob/master/doc/GettingStarted.md