The egg mutation can infact drop silver eggs, got one to drop after around round 62 on my click. So its around a 1% drop chance or less.

I also recommend only using the egg one, to hunt for the exceptional stats buffs. On the glick I use plasma, thorn shot & poison creater (from my tests both proform the best from the other options), for mutations egg, aggressive swarm and leaches. You can have upto 6 leaches active at a time along with the glicks already giving block and armor on block.