Looks OK to me. I am thinking it should only work on living creatures though. Making it Con damage works for me, too.
Say, something like:
Birthing Curse (Su): X times/day, a hippo composite mummy may point to a living creature within X ft? and impregnate them with a scorpion swarm. If the target fails a DC X Fortitude they take 1d4 Con damage per round for 1d4 rounds on the composite mummy's initiative, as the scorpion swarm hatches and devours their flesh. On the final round the scorpion swarm bursts forth, doing an additional 2d6 hit points of damage to the target, and then proceeds to attack all creatures in the area except for composite mummies.
If we just want it to do plain hp damage, how about:
Birthing Curse (Su): X times/day, a hippo composite mummy may point to a living creature within X ft? and impregnate them with a scorpion swarm. If the target fails a DC X Fortitude they take 2d6 hit points damage per round for 1d4 rounds on the composite mummy's initiative, as the scorpion swarm hatches and devours their flesh. On the final round the scorpion swarm bursts forth, doing double damage (4d6) to the target, and then proceeds to attack all creatures in the area except for composite mummies.
I agree about it needing remove curse or break enchantment to cure/stop, like Mummy Rot and Mummy Scourge does.
The proposed 30 foot range seems too short to me, I'd prefer 100 feet. The Mummy has 16 HD, and a Close-Range spell from a 16th level caster has a range of 105 feet.
Maybe also increase the caster level check for the remove curse/break enchantment to 25 or 26, since this is the hippopotamus mummy's schtick and it only gets to do it a few times a day?