FWIW, I had a green haddoni that looked *exactly* like bergzy's. When I bought it, it was perhaps 6" to 8" diameter, and for the first year or so it prefered to stay on the rockwork. As it grew larger and older, it changed its preference from rockwork down towards the rock/sand interface. This is consistent with what I've read somewhere that juvenile haddoni's prefer rock attachment whereas mature adults are dwellers near the rock/sand interface (sorry don't have an URL or article I can reference, it's just something I remember reading years ago -- it may have been one of Shimek's articles though).
I wouldn't rule out mertensii and certainly I would not really argue with you Austin THAT adamantly because you have more experience with mertensii's, but
based on that one picture alone I would not rule out "juvenile haddoni" at this point.
Wager on!
PS. Either way it is a beautiful carpet.
PPS. I know that the carpet I'm talking about was pictured at one point earlier on in this thread. Unfortunately not posted by me as it was posted by AJ_77 whom I had sold the carpet to when it had outgrown the tank I had for it. I'm going to go look and see if it's still showing up (hopefully the picture hasn't disappeared, this is going back something like 4 years now!)

(Edit/"PPPS": ah bummer it's gone.

Only picture of it in this thread is in AJ_77's avatar, in the post on page 2. Oh well, life goes on I guess..)