Floating Stinging Polyps

[Werbo]

I have 15-20 bright green floating polyps floating in my water column. If the floating polyp settles on a SPS coral they sting/burn it. It's a powerful sting.

Judging by the appearance of the floating polyps they are either from my hydnophora (most likely) or possible a euphillia (hammer/frogspawn/torch) corals. I am leaning towards the hydnophora.

What do you think? Has a coral expelled polyps to self propagate and spread (like poccilipora are known to do)?
Or is it my regal tank that I've had for 3 months? I've never witnessed any nipping from the tang.