Hi,
Let see the full sentence.
While the venom in the tentacles of the Portuguese man-of-war is not usually fatal, it can inflict great pain.
When subjects in the first and second clause are the same, we can omit subject and v to be in the first clause (adverb clause --> While ___)
Then, reduced adverb clause should be as follow.
While the venom in the tentacles of the Portuguese man-of-war is not usually fatal, it can inflict great pain.
HTH