How long have you lived here and/or when was the shower last constructed/renovated?

I think it's due to water - especially since the damage is near the shower curb. There may be movement occurring even though that particular area isn't wet. The tiles on the shower wall right above the damage look like they have pushed out a bit also.

However, I am not an expert - maybe a pro will have a less expensive reason for the damage . . .

Shower is leaking and what is behind that mess will look an awful lot worse. You can start with selective demo but prepare to gut and redo the entire shower.

millworkman nailed it. Again.

Something is leaking. I would take a look at how that framed door is screwed into the tile in that corner.

Well, I was trying to put an optimistic spin at the end of my post since it was late last night.

I assumed that the shower has been leaking and the OP needs to determine what the cause is.

It's water damage, just like mine is doing now (hire a pro, they said; not the sort of thing you should DIY, they said). I'm procrastinating demoing out the shower to fix the underlying problem, but I know that's what it is going to take. And when that happens I'll do it myself so I know it gets done correctly.