Call the grout manufacturer and ask them what is causing that effect. Did you mix the grout per manufacturer guidelines? Did you mix too much grout at one time and by the time you got to the bottom the grout consistency could have changed?

Also, it's a good idea to seal porous tiles, etc before grout is applied. Wait for a few hours and apply the grout and after the grout is dry, seal it again.

Water is behind the tile. Grout does not bond to sealed tile, in fact it repells it. You have enough failed attempts at a fix to warrant looking into the problem by removing a tile. What type of caulk did you use? It does not appear to be a waterproof bead. Also keep in mind silicone caulk does not adhere to itself.

I’d bet that a waterproofing membrane was not placed directly behind the tile to level mit water penetration. Systems that rely on waterproofing behind the cement board have a much larger saturated sponge of water that needs to drain.

Tile removed. There is a blue product (waterproofing) behind the board. The photos below show where the glass and aluminum frame join with the tile. Thanks for your help.

Aluminum track could be taking in water and leaking through screw holes into the wall assembly. Is the substrate wet? Is the problem confined to the area beneath the glass panel? If so then the then your problem is at the glass panel juncture.

Blue waterproofing behind the wallboard is a good sign. The gap between the wallboard and the shower base should be caulked per TCNA. A void that size around the entire base can collect moisture. On the bright side there does not appear to be a major problem. The only thing visible is the lack of proper mortar coverage. The mortar was not keyed into the substrate prior to setting.

"Blue waterproofing behind the wallboard is a good sign."

Hmmm. what would that be Blue and behind? . but I do respect CTEC opinion.

Those pics are very small and hard to blow up can you post a better quaility image?

Im not so sure im seeing any blue or waterproofing maybe Denshield.

Cleaned and removed? Silicone requires an absolutely perfectly clean surface to adhere. The only way to break down old silicone residue upon removal is with Acetone. this is a scientific chemical fact just FYI in regards to replacing silicone. It will not bond properly or last without this crucial step in removal cleaning.

In regards to mortar coverage its inadequate and tells me the entire job is likely done that way :(:( half as$#d usual repeats itself all over in my experience.

What I am most concerned about is the beginnings of mold there behind the tile.

You came here with worries and Im confirming I would be worried about the entire job and taking mold seriously. Hoping you have a wrrty. I agree with the aluminum track likely allowing water in behind tile at deck.

Not sure what the blue membrane is but it does show a attempt was made to waterproof. Possibly a pan liner. My first sentence on this thread was.

"My guess would be the issue lies with that detail at the top of the tub deck wall." Which is the glass track.

Tundra thinks he sees sealant. He may be correct. I was not able to establish for sure when I zoomed. So I will trust his opinion.

Perhaps it is a pan liner and i shouldnt have commented LOL. Its very common to attempt to lap a liner over the deck and drop it down into a pan. and the go and screw backer board thru it. I just copied and blew that photo up i think your correct CTEC i retract my sealant statement in gap. just dont see blue but do see mold. Agree w/you and fixer the track may also have screws thru it. what a mess.

Thanks everyone for your very helpful comments. I agree that the void between the wallboard and shower base is large - specifically, what product should be used to fill this void? - sealant type/name?, backer rod?

Re: mold - It looked more like crumbled mortar or cement than mold but will check again. Will spray bleach into space just to be sure. Assuming that if we deal with the water ingress we will address any potential mold issues as mold needs water and substrate to grow.

Re: caulking - have prepped surface properly but wondering which planes/joins do not require caulking. A contractor told me to caulk "everything" however my understanding is that some areas should not be caulked to allow water to escape. Should the areas where the glass and aluminum rails be caulked? Will apply caulk where materials change, i.e. aluminum rail to tile and tile to shower base but not sure what should be left without caulking.

There is no warranty, home is 14 years old and we are second owner - just trying to tackle some of the existing issues. Hoping that we can remove the tiles at the base of the shower, fix up the void between the wallboard and shower base, seal up screw holes, and all will be good :)

Learning as I go along so thanks again everyone for your help!