I'm missing a metaphor: the one where something initially co-exists well with other things in its environment, but eventually grows so big without intending to that it absorbs or squeezes out everything else around it. I'm sure it's not a white elephant: that implies that it was foisted on the host and that it's useless as well as resource-sapping. I also don't think it's "a cuckoo in the nest" I take that to mean something which invades the environment with the intention of eventually taking over.
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