Seven thousand years ago, the Yami ancestors of Maori on the island of Formosa (now Taiwan) gave the name omo-omotan to moss growing on the side of rocks and seaweed on rocks at low tide.
 
Their descendants who sailed to Philippines 4000 years ago used alomot, and later lumot, for moss (which was inedible); but used limu for seaweeds (which they ate), both on tidal rocks and in deeper water.
The Austronesians then kept the name limu for seaweed as they migrated to north-east New Guinea, then the Solomons, then Samoa and Tonga.

Islanders in the Pacific adopted different dialect words so that when they visited other islands, their hosts would know for sure where they were from.

So in the Tahitian, Tuamotuan and Cook Islands, they lowered their tongues a little, rolled them like Scottish do, and called seaweed /r/imu. English speakers keep their tongues flat and say [ɹ̠]imu.

When voyagers from the Tahitian and Cook Islands colonised Aotearoa, they found it covered in huge trees with branchlets drooping down and swinging back and forth just like seaweed, so these named these trees
And to avoid confusion, seaweed here is usually called rimurimu.

When you stop for a rest later on, you can read some MORE fun facts about rimu.