I'm just speculating based on a spawn of Xol (Mormu) being in this general area. I think the Hidden Swarm is really a branch of Crota's spawn because you see them as the first to attack earth, which were soon followed by an attack by Crota's legion.
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But there is also Gotra and Vurok, both Eir Spawn. The Spawn of Crota are kind just Crota's personal servants and disciples, Hive that have been born and fed under Omnigul. The Hidden Swarm have unknown origins