I was going to say if the silicone looks in good nick, mold notwithstanding; buy and use HG mould spray, and squeegee shower walls after showering) it's more likely that the grout has failed
Buy a grout rake; looks a bit like a toothbrush with a bobbly metal blade, and use it to remove all the grout around the affected area, then mix up new grout per the instructions on the bag and use a grout spreader (kinda like a rubber blade with a handle or a sort surfaced plastering trowel) to push the grout into the gap like you're buttering toast; get it all over the tiles too, then round it up into a ridge and push it into the gap again, finally crossing over the tiles edges in a diagonal direction, set a lump of grout aside and keep checking on it. When it starts to go firm, but is at a stage where rubbing it with wet finger "dissolves"/smears it over your fingers, wipe the wall down gently with a damp sponge that you clean regularly- the aim is to wipe grout off the tiles and also just a little bit out of the gap to smooth it and give it that typical U shape
No harm in giving it a go, like I say if you can butter toast you can grout a wall and if you make an ass of part of it you just rake it out and try again