I had about 20 of them on my candy cane trumpet coral. They are tough to see if the coral is out of the water, because they shrink down and "de-flate". They are still hard to grab and get off, even with tweezers. What I did was put the coral (rock, or whatever is infested with the hydroids) into a separate container with some tank water under good lighting, and try to pull them off. Tanster recommended using some sort of Kalk paste to brush over them and smother them out. Try that if you've got it handy.