It could be a wonderful thing for both parties if presented properly. He recreated the entire game by himself thinking it wouldn't be plagiarism. However, just like a college essay, if you write down all the sentences yourself but the use of the words within these sentences are from other people's work, we consider it plagiarism.
Who cares whether it's "plagiarism"? Plagiarism isn't illegal, and in many contexts, it isn't even wrong. Plagiarism is an academic concept, not a legal or business concept. Ever major computer company has "plagiarized" in their products, i.e., taken ideas from others without acknowledging the source, and that's OK. In fact, this game case is probably not even plagiarism, since plagiarism means using material without acknowledging the source, and they may well have done so. On the other hand, a lot of legally infringing activity is not plagiarism at all, so not plagiarizing does not protect you from legal claims.
In business, what matters is copyrights, patents, trademarks, and contracts. The game could be taken down because Apple controls their app store and can do whatever they want. If Aquatica were sold outside the app store, the flOw developer would have had to go to court and claim copyright, patent, or trademark infringement.
Is that cloning or theft?
It's theft when there is a law against it. Did the game infringe copyrights, patents, trademarks, or did it break contracts? If so, it's "theft". If not, it's not "theft" in the usual legal sense, although it may still be plagiarism.
Given the similarity, I suspect that the Aquatica developer did commit copyright infringement, but that's really for a judge and jury to decide.