Could I simply seal over this toilet leak?
Yes, you could seal between the tank and the nut. You'd have to drain the tank and have both parts dry.  But there's a better solution that isn't any more difficult. 

That nut is on the bottom of the fill valve column. Inside the tank there is a rubber washer between the base of the column and the tank.  That's where your leak is. If the rubber washer has deteriorated, replace it.  If you want to use silicone, do it there-- you can coat both sides of the washer to seal with the valve column base and the tank.

I did this before and it worked for me.