The slowdown from an otyugh's attack is a side-effect of the disease they inflict, not from a magical 'slow'. The disease-slow can't really be blocked without blocking all disease effects; on the flipside it can be cured with Cure Disease or blocked by anything that prevents disease.
And that's setting aside the whole Rules Lawyer discussion over whether immunity to slow (or free action) should protect against slow-via-disease in the first place.
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And that's setting aside the whole Rules Lawyer discussion over whether immunity to slow (or free action) should protect against slow-via-disease in the first place.