Okay, I'd favour the same AC increase and HD decrease as the Giant Croc Mummy proposals above.
That is,
Armor Class 15 or 16 and
Hit Points 184 (16d12 + 80).
It's got the Giant Croc's movement speeds, a Fasolasuchus is faster on the ground.
Maybe
Speed 30 ft., swim 15 ft.?
Shouldn't it be DEX 10 (+0) rather than 9 (–1) since the other mummies have Dexterity two lower than the living version?
That'd give it the Stealth +4 it's listed with.
As for the
Actions, the bite and tail need the same damage tweaks as the Giant Croc, and its Death Shake should carry the rotting curse too.
Oh, and it should have "fasolasuchus mummy" rather than "crocodile".
Like so:
Rotting Bite. Melee Weapon Attack: +11 to hit, reach 5 ft., one creature. Hit: 23 (3d10 + 7) piercing damage plus 14 (4d6) necrotic damage, and the target is grappled (escape DC 19). Until this grapple ends, the target is restrained, and the fasolasuchus mummy can't bite another target. If the target is a creature, it must succeed on a DC 17 Constitution saving throw or be cursed with mummy rot. The cursed target can't regain hit points, and its hit point maximum decreases by 14 (4d6) for every 24 hours that elapse. If the curse reduces the target's hit points to 0, the target dies, and its body turns to dust. The curse lasts until removed by the remove curse spell or other magic.
Rotting Death Shake. If the fasolasuchus mummy has grappled a target smaller than itself with its jaws it can savagely shake its head. The grappled creature must make a DC 16 Constitution saving throw, taking 16 (2d10 + 5) piercing damage plus 16 (2d10 + 5) slashing damage and 28 (8d6) necrotic damage on a failed save, or half as much damage on a successful one. In addition, the creature must succeed on a DC 17 Constitution saving throw or be cursed with mummy rot. The cursed target can't regain hit points, and its hit point maximum decreases by 14 (4d6) for every 24 hours that elapse. If the curse reduces the target's hit points to 0, the target dies, and its body turns to dust. The curse lasts until removed by the remove curse spell or other magic.
Rotting Tail. Melee Weapon Attack: +11 to hit, reach 10 ft., one creature. Hit: 16 (2d8 + 7) piercing damage plus 14 (4d6) necrotic damage. If the target is a creature it must succeed on a DC 19 Strength saving throw or be knocked prone. In addition, the creature must succeed on a DC 17 Constitution saving throw or be cursed with mummy rot. The cursed target can't regain hit points, and its hit point maximum decreases by 14 (4d6) for every 24 hours that elapse. If the curse reduces the target's hit points to 0, the target dies, and its body turns to dust. The curse lasts until removed by the remove curse spell or other magic.
I think that's everything and, if I'm reading the CR Calculator right, still comes to Challenge 10.