Translation:I saw the green dragon coming out of the cave.
I wrote "leaving" the cave instead of going out and was marked wrong. Whoever set up the English for this is not a native speaker.
I also tried, just for fun, other viable options in English, and none were accepted!!! Namely: exited from the cave, and departing from the cave. You have not done a thorough job, since all three of these very common synonyms are perfectly acceptable translations.
It's not wrong, and neither is "I saw the green dragon come out of the cave", which DL also rejects. As ToddKedes pointed out, there are problems with English translations in this module. In fact, there are problems with English translations across the course, especially towards the bottom of the tree. IMO that's the single biggest problem with DL Hebrew, at least as of May 2018.
To be fair, the same thing happens in other DL courses, too. Modules towards the bottom of the tree tend to be proofread a lot less carefully than modules towards the top. Most users never get that far, so they get lower priority. In a course that only recently emerged from the incubator, and that is entirely volunteer-run, that sort of triage makes a lot of sense.