"There is something moving in my soup" should be allowed. There is no need to require the word "that's" in the sentence.
It must be coming from "ada yang"; which from what we've learnt so far, I would have translated as "is which is" which makes no sense.
I can only presume that it works out to mean something more like "there is (a thing) which is" - and thus ends up simplifying to "something"
To make sense of the words used i put it as ada 'is' yang 'that' "Is that movement in my soup?" Which is quite accurate but wont be accepted by duo. Its just this is one of those phrases which simply doesn't translate well into english. I would prefer my rendition because it's accurate with ada yang in meaning and we can flex with verb and choose either 'moving' or 'movement'