I would just inspect the other plants thoroughly every so often, they may not even be susceptible to that scale. Do you know the names of the other plants you have?
If you have a large tub you could try inverting the whole affected plant in soapy water, say 1% hand dishwashing liquid to water (not dishwasher machine detergent). Wait a couple of weeks and then do it again. I don't know if that plant is damaged by soaps, maybe someone else does. You might want to take the risk anyway.
