It's missing the cheek spot for diadema (which you can clearly see in Andicca's pic) or the stripe for metae ...
These are not distinguishing characteristics between these 3 species. All 3 have a cheek spot, as well as a horizontal stripe......though these markings are not necessarily expressed. I can find pictures of all 3 species, with or with out a cheek spot! Generally you might expect, the more mature the specimen is, the less likely it will show it's cheek spot.
Can be difficult to distinguish these species. But if you look at the markings on the fins, seems to be a match with pictures of Aequidens diadema ......and not a match with either A. pallidus nor A. metae. Though I am not certain that is a reliable way to distinguish these species.