I was using this guide to fix that annoying "Monster stuck in the wall" error with yellow sprites around them. But, when using the fix to my headcrabs, they don't attack (and move) until player does damage to them. I even tried to place info_node's below the headcrab and scripted_sequence, but it didn't help.
Where exactly are you trying to place your headcrabs that this fix is necessary? The pollen sprite means an NPC is too close to the level geometry. The fix is to move them away from the surface they're stuck in.
That guide (I only scan read it) seems to suggest using a scripted_sequence to move them into position for scripted events. That's fine if you want the NPC to remain in that position, but not for the sake of them acting like normal enemies. They will remain looping in their idle animation until triggered to leave their idle state.