So I picked up one of the Pencil Blow Torches at Home Depot.
Will try on one rock to see if burning the Hydroids will work. Pencil flame may be more precise than a regular torch.
I am being cautious on safety since burning live rock could release TOXIN smoke. So I'll doing the burn off in the garage with a Fan blowing away from me.
Also I built this dremil power tool attachment during a Algae Bloom I had last year.
It might be good to blast away the hard to reach Hydroids. I have to pull it out storage an try it.
Here is a you tube movie (fake advertisement) I did a while ago
https://www.youtube.com/watch?v=Z0FJ9_R6Fyg