Tomonts cannot encyst to macroalgae, but free swimmers can inadvertently be in a droplet of water on the plant. However, rinsing it thoroughly with DT water (or clean saltwater) will "wash away" this threat.
Now if you wanted to be even more cautious, isolating it to a fishless frag tank will starve any free swimmers. 2 days for ich - but 15 days for velvet since dinospores can use photosynthesis to survive longer without fish flesh to feed upon.
Chaeto presents a special challenge because that often comes loaded with pods. Pods are crustaceans with exoskeletons that tomonts can encyst upon. So, I would use a coral dip (like Bayer) on chaeto, followed by a good rinsing with DT water.