If you have at least 1 in your inventory, I think you are supposed to use it before/while fighting Fallen to earn extra Glimmer. The Ether Seeds page doesn't seem very clear on their use, either.

Thanks, but somehow it doesn't work. I was in the Divide behind a corner, there was at least 15 Fallen and it didn't work. The Black Wax Idol, the Blue Polyphage and the Resupply Codes work well but not the seeds.

If you have a perk on your Ghost, you can earn glimme from Fallen when equipped. Have also tried checking your timer? Sometimes it just doesn't work.

How do you get that Ghost's perk? Is it from a shell? if yes, from which shell?

There's no specific shell that has it.  WotM ghosts will sometimes have it but it's not guaranteed.  You need one that has the "Ether Seeds" perk.

Some of the Vanguard and Crucible shells have the perk. Try getting them out of packages or buy them from your class vendor (Zavala, Cayde or Ikora).

Thanks. Cayde-6 was selling one of these shells but I didn't have enough Legendary marks. Is there a simple way to get some ? I'm level 27 and I don't Raid.

If you're only level 27, there aren't many places you can get Legendary Marks. Dismantling legendary weapons and armor (purple) should give you some, so if you have any that you no longer use, I'd do that. Otherwise, you really need to get to level 40. Continue doing the story missions. You should be able to get to at least level 35 that way. Then start doing heroic strikes.

Thanks!