Egg Sack?
 

Today while i was attempting to catch an emerald crab that im trying to sell, I noticed an "egg sack" white to brown in color hidden underneath one of my rocks. Does anyone have any idea of what this is? Do I need to kill it or let it grow and hatch?

I vote let it hatch, probably a bunch of snails or something.

hoping their snails, but then again i would rather be safe than sorry..

You could float them in a bag and change the water daily

I cant really think of anything that might make you sorry. Snails lay eggs all the time, usually they get gobbled up by your fish before they hatch. Even then most of them will get eaten before they grow at all. My last tank had a bunch of snails hatch and I only had an algae blenny in there. pretty soon my tank was covered with baby snails. Between them and the brittle stars, feeding made the whole tank look like it was crawling (creepy...).

Bottom line i couldnt see them hurting, they will probably just end up as a free snack for your fish.

try googling images of sponges....some sponges kinda look like egg sacks....

I've never had any sponges in my tank so I can't see it being that..but time will tell though

Pretty much all reef tanks have sponges...they like to be in the dark so you don't usually see them on the rocks. They are usually in shaded areas or in the sump/overflow box.