I made this Star Moss farming guide to help out players who want to farm the herbs instead of buying them from the Auction House. You can pick Star Moss with BfA Herbalism skill 1, but the rank 2 and rank 3 requires higher Herbalism skill. Higher ranks allow you to gather this herb more effectively, so you will get more herbs overall.
Star Moss only grows on buildings or ruins, so you should look for it on the walls and not on the ground.
Star Moss
Star Moss ranks
You will learn rank 1 from your Herbalism trainer at Daz'Alor / Boralus.
Rank 2 is a reward from 
Here In Spirit. This quest unlocks at BfA Herbalism 50 and is given to you by your Herbalism trainer at Daz'Alor / Boralus.
Ranks 3 is a reward from the quest Ghost Busting. This quest unlocks at BfA Herbalism 145 and is given to you by your Herbalism trainer at Daz'Alor / Boralus.
How to speed up farming
Battle for Azeroth introduces three new items that you can use to speed up farming herbs. You should use these if you can find them cheap at the Auction House, or craft it yourself if you can.
[Monel-Hardened Stirrups] - You can gather while mounted.
[Monel-Hardened Hoofplates] - Increase mount speed by 20%.
[Coarse Leather Barding] - Prevents you from being dazed. (mobs won't dismount you)
The one I would recommend the most is [Monel-Hardened Stirrups], which lets you gather herbs while mounted. Every mob scales to your level in Battle for Azeroth, so it will be hard to avoid the mobs attacking you while you try to farm herbs. If you have the 
[Sky Golem] mount, you can use that instead of the Monel-Hardened Stirrups
If your class has a tanking spec, you should switch to that while farming, so you won't get dazed and you don't have to use Coarse Leather Barding.
Don't forget to enchant your gloves with 
[Enchant Gloves - Zandalari Herbalism] or [Enchant Gloves - Kul Tiran Herbalism]! This enchant brings gathering time down from 1.5 seconds to 0.5 seconds. This is really important if you gather while mounted because mobs can still interrupt your casting.
Vol'dun
The route will give you the most Star Moss overall.
Drustvar
There are a lot of mobs guarding herbs at this route, so you might have trouble here if you don't use the enhants I recommended above.
Vol'dun
You will mostly get Akunda's Bite here, but you will get some Star Moss, so you can use it as an alternative.
