What do you mean the "tank seal"?
Do you mean the actual RTV between the two pieces of glass?? The joint itself?
Or what has squelched out when being made and rounded off in the making if the tank??
If the former, and you can see where the RTV has not filled properly,
and an airtrack is showing, you may be able to squeeze RTV back thru to fill it.
If it's failing, strip it down and start again.
If it is the bead, which I suspect it is.
Just cut out all the beading and redo it.
That'l pick up the leak.
The green you refer to, as all the seam is not coloured, my guess,
is that it is algae growth that has got under the bead and it will slowly spread and lift all the bead.
Sooooo, my best bet is to strip out all the internal corners,
I use a piece of freshly cut glass, it has very sharp edges and gets into the corner to do the removal,
clean back to the glass, removing all the RTV and any sign of the algae and apply the RTV,
and pointing it off with a wet finger.
Leave 24 hours then fill.
Alan
NZKA 104