Also, I have a theory about how something can be weak to both. First light is restore and dark is decay (at least, the crystals are). Now, if you restore a crawler to its previous stage, you get a very weak larva. If you decay a crawler, (which they could be very susceptible to if they have a short (natural) lifespan.) They'd probably grow very fragile.
But if you want a more direct reason...
Imagine something that needs light to survive, but sunburns very easily, like the common computer geek.