Question

How is my Grimer immune to leech seed?

Comments

Does it have the ability liquid ooze? You need to give more details.

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it's ability is Gluttony.

Answer 1

If your Grimer was an Alolan Grimer, which is a Dark type Pokémon, it would not be affected by status moves used by a Pokémon with the ability Prankster.

As of Generation 7, Dark type Pokémon are immune to status moves, such as leech seed, used by a Pokémon with Prankster. Were you fighting a Cottonee? Cottonee can have Prankster as one of its abilities, and therefore a Cottonee with prankster would not affect a dark type like Alolan Grimer with a status move such as leech seed

If you were not fighting a Cottonee or a Whimsicott then I do not know what was going on.

Source: https://m.bulbapedia.bulbagarden.net/wiki/Prankster_(Ability)

Comments

Whoops. It was  a Petilil but I am going to BA anyways as this answer is better than Rex's.

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Was your grimer affected by Forests Curse?

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Well, it never used Forest's Curse.

Answer 2

It's not

None of Alolan (or regular) Grimer's abilities indicate that it's immune to Leech Seed. As it's not Grass type, it is affected by Leech Seed. The only ability that won't let Leech Seed work is Magic Guard.

Therefore, if and only if your Alolan Grimer has Power of Alchemy as its ability and a Pokemon with Magic Guard in your team fainted before, it makes sense for that to happened.

But that's not as likely as this:

If you were hit by Leech Seed, but took down the foe the same turn, then there is no foe to drain the HP to, and so your HP doesn't go down. Simple as that.

Source: Personal experience.

Comments

But thats incorrect. Once a Petilil used leech seed too my Alolan Grimer first turn outspeeding it and the message said "It does not effect Grimer..."  and it took 2 turns to defeat the Petilil. And the only pokemon that fainted was a pokemon that doesn't have magic guard.