sculk is based on sound, JoshBlueMoon's hollow is based on movement, and the decayed is based on sight, instead of sensors or shriekers, there's decayed sighters that mesmerize you to look at it and "take a photo" to alert the Stalker. The decayed is a naturally made organism that looks like a mash-up of rotten flesh and mold, which it is.
instead of needing something to kill, it needs a decayer to spread, decayers are entity blocks that spew spores as a defense mechanism that convert nearby blocks into decayed blocks (if done enough times), if a player is near one and it spews spores, the player is given the withering effect. It spreads decay by itself because it is a fungus-like species, it spews spores out of the holes in the top area of it's body to keep intruders away, or die if they still try to prevail.
The sculk sensors of the decayed, they need intruders of the biome to get close (3 blocks) so they can hypnotize them until they "take a photo" and neurologically send it to the Stalker, the player is given blindness for 5 seconds after. When trying to hypnotize you, your camera slowly turns toward the decayed sighter's eye, once you look at it fully, you can't look back until it flashes you.
The warden of this biome, it can see, but it can't hear or smell. It is summoned once a decayed sighter has activated, it slowly creeps up behind you, if you look at it, it will disguise itself as a dead body, if you try hitting it, it will disintegrate itself and materialize itself somewhere else, which in its lore is it killed an enderman and corrupted the ender pearl, putting it in the hole in its torso. It has 5 hearts (10 hp) due to it being rotten flesh and mold.