hmm, my concern is that what are those things.... & if there is a way to prevent them.
Those are scale insects. Usually ants is the vector for these bugs.You can physically remove them if the infection is not serious. It will take a lot of time and patience to do so. There are some people who advise dipping the plants in a very weak solution of dishwasher soap, but I'm against this as I find platies are rather fragile at that kind of treatment.
Otherwise tou can try systemic insecticide like dithane. It works well for me. You can follow the instruction advises there. Try at weaker solution first to se how it owrks. You can tell the bugs is dea when you can easily peel it of after a couple of days.
Hope this helps.