tl:dr is that Pyke should not be allowed to heal for his Grey Health, if he has taken any damage (excluding damage from items and neutral monster buffs) from an enemy champion within the last second.
I'd been picking Teemo as a counter to stealthy champs like Evelynn, Rengar, Twitch, etc. Shrooms put sight, but not "true" sight, on their victims. But I'm watching my replays and seeing Pyke players literally rushing ahead of their allies to "clear the Shrooms". He's able to both take damage from the Shrooms, and also gain Grey Health, AND also heal for all that Grey Health, all simultaneously. So basically he just runs around intentionally stepping on my Shrooms, because they don't reveal him, and he's continually healing for most of the Shroom damage, all at the same time.
That's because Pyke "rapidly heals for his Grey Health" while the enemy is unable to see him. And since the Shrooms only put regular sight on him, and not true sight, he remains camouflaged even while stepping on the Shrooms. So each time he takes a tick of Shroom damage, he almost immediately heals for most (81% at level 18) of that damage, and that's not counting any further reductions he gains from his magic resistance. So he basically takes zero damage from the Shrooms.
I think that if Pyke has taken any damage (excluding damage from items and neutral monster buffs) from an enemy champion within the last second, his ability to heal should be temporarily disabled, EVEN if the enemy team has no sight of him. This would NOT disable his ability to gain Grey Health... only his healing off that Grey Health would be temporarily disabled. So if he steps on a Shroom during his W, he would likely have to wait till his next W comes off cooldown, before he could finish healing for all his Grey health.
Periodic tick damage from items like Liandy's Torment or Corruption Potion, or neutral monster buffs, would not disable Pyke's heal. But summoner spells like Ignite, and ability damage like teemo's Shrooms or blasts from Xerath's ults, would definitely disable Pyke's healing for 1 second. This would mean that if you can keep blasting the Pyke with damage, he could die even during his W, since his healing will be disabled as long as you keep damaging him every second.
If Riot really wants to have a hard counter to Teemo shrooms, I'm all for that. I just don't think Pyke should be it. As a Teemo player, I'd be totally okay with something like Nami's ult being able to clear all the shrooms it touches. That would make a lot more sense. But I feel like stealthy champions like Pyke should be countered by Teemo shrooms, and not the other way around.