closed as unclear what you're asking by Blavius, macraf, broccoli forest, snailboat♦ Dec 26 '16 at 2:05
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This is just a standard -てくる formation combined with -ている to express that it's a continuing process. This might seem awkward in the abstract, not so much when you think about context. 「歯が生える」 means that "teeth are growing." Conceptually speaking, teeth aren't a part of you (at least as teeth) until they grow in, so adding on a -てくる to this makes the nuance more along the lines of "teeth grow in[to one's mouth]." -ている here just means that the process is continual, so "teeth are growing in." Putting it all together, you get something along the lines of "Be careful of the Venomous Tentacula. Its teeth are coming in."
Since I had no idea what a 毒触手草 was I did a quick Google search and found some Harry Potter thing. Googling that along with "teeth" led me to this page, so if you have similar questions in the future I'd recommend digging around on Google for a bit since Pottermore is pretty popular.