Sahuagin have the Blood Frenzy trait:
The sahuagin has advantage on melee attack rolls against any creature that doesn't have all its hit points.
If a Sahuagin casts spiritual weapon, does the melee spell attack from qualify for Blood Frenzy?
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Spiritual weapon states (emphasis mine):
When you cast the spell, you can make a melee spell attack against a creature within 5 feet of the weapon.
As a bonus action on your turn, you can move the weapon up to 20 feet and repeat the attack.
It is clear that the caster is making the attack despite the fact that the weapon may be far away from the character itself.
A melee spell attack is a melee attack, so it satisfies the requirement for Blood Frenzy if the target has missing hit points.