"It is not necessarily bad."
It is the particle は but is called "contrastive は" in these links.
To everyone wondering what the ha means it is a way of partial negation. http://www.guidetojapanese.org/forum/viewtopic.php?id=1686
Yeah, Duolingo does like to do that. Here's some information about how this is used:
Scroll down to: SPLITTING く AND ない WITH PARTICLE は FOR CONTRAST
"You might use this when you want to say that something isn't bad, but imply that it isn't exactly good either, without stating it explicitly."