Looks fine to me, they'll swell up like that sometimes. He could be moving around looking for a better spot. I had one of mine do that recently and the next day it split into two. I would just leave it alone.
I have been keeping Nems (have 8 now) for several years and I agree with Ygon. Nem looks fine to me.
As for causing a crash, I have heard that but never seen it happen.
If it was dying yes I would remove it. Usually they'll look like they are melting when that happens, that doesn't look to be your case but keep an eye on it.