I've found that the skinned surface is more resilient than the interior (carved) foam when exposed to light/heat/humidity/UV, but if it's a small spot, it isn't likely that it will deteriorate beyond the exposed portion.
Great stuff survives outside in full exposure to the elements and sun for years. It looks a little beat up after a while but stays intact. I think your spot in a frog tank should be just fine. You can also just hit that spot again.