Malenti have the typical sahuagin natural armor bonus
The Monster Manual entry on sahuagin says that mutant sahuagin malenti "have a swim speed of 40 feet, can remain out of water for 1 hour per point of Constitution, and have freshwater sensitivity and light sensitivity [but] have no natural attacks [and] are otherwise identical with sahuagin" (218). Thus a malenti's sahuagin identity should extend to a malenti gaining a +5 natural armor bonus just like any other sahuagin, but I totally understand the skepticism: that's a pretty crazy high natural armor bonus for a creature that "looks just like an aquatic elf" (and could be a giveaway as to the malenti's identity in the right situation).
However, to confirm, at least three published malenti gain the natural armor bonus of the sahuagin:
- City of Stormreach has K’shegla, a malenti of legend cleric 7 (155). He has a Dexterity score of 20 and gains a +5 increase his natural armor bonus from the template monster of legend (Monster Manual II 213-4). His AC is listed as 20 normally and when flat-footed and 15 against touch attacks. With no other sources of armor, the natural amor bonus of the sahuagin is the most reasonable source of this unmentioned-by-the-text bonus to Armor Class. (Also, K’shegla's illustration looks nothing like an aquatic elf, but that's hardly his fault.)
- More telling, Secrets of Xen'drik has the unfortunately-named Korel, a malenti expert 1/rogue 2 (64-5). The sources of Korel's Armor Class bonuses are actually detailed: she receives a +3 bonus from her Dexterity, a +2 armor bonus, and—for no other reason that I can discern except for her malenti heritage—a +5 natural armor bonus to Armor Class.
- The Web adventure "Shoals of Intrigue" has Medjil, a malenti Drd 4 (6). Her AC 18 (touch 12, flat-footed 16) is a product of her Dexterity score of 14, her bracers of armor +1, and—presumably—her sahuagin identity, no other source conveniently explaining her +5 bonus to AC.
While three is a really small sample size, the malenti is a fairly obscure monster—a 1-in-100 mutant offspring of an evil underwater race—, so having three sources that agree that the malenti receive the sahuagin natural armor bonus should, I hope, be enough.