Would it help to suppose plents generally have green blood for the same reason
green-blooded skinks have green blood? The green-blooded skinks still have hemoglobin, but their blood is green from a breakdown product of cells thought to protect them against malaria. (
Other species. can have greenish blood or use of the pigment, for the same reason.) Or should they use chlorocruorin?
The Xenowasps genus groups, as they are widespread and some of their members drink blood, could have a mosquito-like role in spreading malaria-like diseases. (Historically, the Bloodbee could have done this, and it's possible the mucus-eaters of the Xenobees genus group could spread disease similarly.) Xenowasps, Xenobees, or their ancestors spreading diseases probably wouldn't have been the cause of plents having green blood, but their widespread status now may discourage plents from evolving away from the green blood trait where they co-occur.