aquarium seam

Not sure if [EMERGENCY] is right, but I hope not so I'll leave it off... I have a several years old AGE tank with half inch low-iron glass. Today while doing some cleaning, I noticed this seam, and how far back the silicone has receded due to algae and whatever else it encounters in the tank...

I'm new to the hobby and picked up this 125g used tank a few days ago. The gentleman I bought it from still had water in it until the morning I went and picked it up. Now that I have been able to look it over, I've noticed areas in the seams where he must have hit the silicone with the algae...

So I have a brand new 150 gallon Sc aquarium. I received this tank and it looks great. Before I add water, I noticed a seam on the back that had a 4 inch bubble. However it's not between structural seam of the two panels of glass. Those seams are perfect but it's the lining of the silicone...