that's normal and nothing to be concerned with.
look at any glass tank that has been up for months and in light and you will see the same.
As far as I know, most silicone with get opaque with age.
What you want to watch for are any areas where it looks as if the silicone is actually lifting off the glass at the edge of the bead, that can be a potential disaster.
Is that a crack/chip in the first pic or air trapped in the silicone bead?
If it's air in the silicone seam, return the tank.