Leaks are no fun , but the best news is that you located the leak ( the hardest part).
You will need to empty tank, so have a couple of Rubbermaid tubs ready. Make sure theyre C L E A N !!!! Transfer everything to tubs and utilize water pumps and lighting.
flush area where leak is WELL. Then you will need a razor and peel ALL silicone away from the area extending to removal of silicone at least 4" in both directions. Once scraped away, clean area well with rubbing alcohol. Wipe with lint free cloth and have aquarium sealant ready, not plain old silicone. Dow, aqueon and all glass are great sealants. Apply generous bead into seam and smooth out with a plastic spoon o0r large popsicle stick and let it dry at least 36-48 hours. Once dry/cured, water test with freshwater and allow to sit 6-8 hrs. Once No leak confirmed, repack your tank with 80% new mix and add from tubs, the remainder needed to fill tank.
I owned a retail full line pet store for 11 years and successfully sealed 51 tanks for customers and even one of my own.