No. They are not protozoans. Any google search that suggests such is wrong. Only trust published, peer reviewed, information when doing web searches or you will be misled often.
Not sure a freshwater dip will solve the problem either.
I had a small patch of colonial hydroids come in on some ARC liverock (Atlantic Reef Creations). Ignored for a while and then they spread. Ended up taking that rock out plus 3-4 pieces of pukani, broke off affected areas and discarded half the rocks (bleached in sun for a few weeks now in a QT back up tank). Also had to sacrifice a hammer coral that I liked as the hydroids were all over it.
Just FYI, if colonial, they actually grow via runners in small channels within the rock surface. So if you cover with something they just pop up on the edges eventually.
If you have another type maybe you have some more options. Don't know. Better to tell us what type you have or add a pic of them for better advice.
