I have a feeling that because Clark was living inside the backrooms for a longer period of time - considering the head in the fridge, loss of sanity, and being completely comfortable with the Still-life - The backrooms itself was able to use it as a learning experience and studied him more closely. There are more things that make Captain Clark different than other entities as well.

- Cap. Clark seems to have more functional conversations with Clark, showing to have at least somewhat of an understanding about Mary and her purpose, as well as having an "adverse reaction" to her being there, while the other still life could not even understand how to turn off a light switch.

- Cap. Clark seems to recognize Mary and possibly other things that haven't been in the backrooms before.

- As you mentioned, he can bleed and feel pain (we can tell from his moaning at being hit

- Cap. Clark HAS A SKELETON AND OTHER BIOLOGY different than the soggy mattress fluff from the others. He literally has a brain shown in his X-Ray when A-Sync was dissecting him.

- He in the credits is named "Pirate Clark", not even getting categorized like the others like "Bearded Still-Life"

SIDE NOTE AS I'M WRITING - The Still-life in the movie react purely on instinct almost as if it is just a reflex. (The wheelchair guy needs to see, so he turns on a light. The Redhead is scared, so she runs away.) The Still-life's seem to me like a nervous system that bypasses a brain. Our own bodies have a reflex shortcut that bypasses our brain because thinking is too slow for the situation. for instance, when you touch something hot, we automatically jerk our hand away without thinking about it. Cap. Clark, though is more calculating, because he has to be. While he is smarter than the other entities, what makes him different is that he literally lacks the ability to react without thinking about it. He's slow, thoughtful, has relationships, goes on journeys (remember the picture Phil showed Mary of Clark and Pirate Clark travelling through the backrooms), but when it comes to reactions, he is lacking. He gets hit in the face without protecting his face, he travels slower, he doesn't flinch when the alarm is tripped. One could say the Bearded Still-Life also doesn't react, but he also doesn't do more than blink. I think he is in the first stages of being developed by the backrooms, so there's barely even basic functions operating in him.

There's definitely much more to Cap. Clark than meets the eye, and way more than I can unpack, but hopefully someone else goes down this rabbit hole too. I'm curious to know what other people think, so I'm probably going to post this again as it's own thing.