This is not ideal, but may not be particularly deep yet…
Black silicone hides these seam failures very well… Can you try shining a bright point light source (such as a phone flashlight) right up against the glass around that area in the seam to see if any distinct areas of the black silicone start to stand out as a non-uniform lighter grey (like a void inside the dark silicone transmitting more light)
getting a few more photos from several different angles would help if you do see any irregularities around this sand filled seam segment!
Unfortunately, silicone typically doesn’t work well at all unless the tank is dry… it cures via moisture, and won’t stick well to old silicone…
Well, you have a Red Sea tank so yeah, regardless you should be concerned.
However, that doesn’t look horrible. Looks like some sort of a nick in the seam.
Hi There Recently I worried about tiny worm trails under the silicone. I sent a few pics to Red Sea who said drain the tank - and sent a replacement within 5 days. So, if you are under warranty still, send them photos.
Having gone through a leak that ended in an upgrade, I would be concerned as well. IF you decide to move forward as is, just have a plan drawn up to handle the situation should a leak occur.