"A girl is falling."
I'm sure this is a common problem for native english speakers. In many languages, like Esperanto and German, the present tense of a verb is the same as its present participle. That is to say that, "He walks" and "He is walking" would be the same, "Li promenas". You are semi-correct in saying that "estas" is used for descriptions. I just started learning Esperanto recently, but I believe that there is a different word for describing feelings/states of being, as in "I am feeling good". Can someone confirm this for me?
I believe it's safe to assume that - English seems to be less common in that it separates "is ???ing" from "???s." And considering Esperanto is supposed to be a simplified language, I think that furthers the evidence to support that your statement is correct. I'm a learner too, so I don't know 100%, but it sure looks that way!