Thats' nasty!

Caulking it will not do the job - you can't seal out the water - the marble floor tile is porous and is too rough to seal. Water will get under there. Is there a version of that tub on legs - just enough to get it an air gap so the water drains and you can hose out under it?

I assume that is faux marble porcelain tile. Standing water is usually the issue. How was your slope done?

It is a ceramic tile not real marble. It’s slightly pitched to the shower drain the the real slope is at the drain. The “standing water” issue seems to happen when we run our steam shower. A regular shower doesn’t put that much water near the tub. We also concluded that the tub apron was not properly screwed to the tile floor based off the manufacturers specifications, but we still need to seal it because no matter how perfect your tile installer is, it won’t be 100% level.

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Did you hire someone to do this for you? If so, especially since it's only been 8 months, call them and have them come out to inspect. This just isn't right. They must have done something wrong.

I'd be caulking that - but........ you are never going to be able to get a good bead behind that tub and seal it. The steam shower / steam / condensate will get everywhere and sit back there / get under the tub. The condensate will run down every inch of the outside of that tub and find whatever tiny weakness in the caulk you have.

I've never heard of doing a steam wet room - that would appear to be a condensation surface area nightmare. Aren't steam showers usually smaller and self contained?

It looks like a fairly large room. Could you separate the tub from the shpwer area? A proper pitche slope leading condesation and running water to the drain without obstruction is important in a steam shower. Is it only water proofed or did they vapor proof aswell?

Kohler makes steam generators for all sized showers. We used Wedi board (same system as schluter system) to water proof the entire area. We then applied a custom made rubber floor (due to size) and concrete before installing the tile.
We have determined a fix. It turns out the shell was never caulked properly which is why the water got under it. Previously we used the steam shower three times a week but in the past 3 months we have used it twice total. I don’t think the steam was the underlying issue, but it definitely does create a lot of moisture. We have some perpetual maintenance items but we’re okay with it.

Thanks for all the input!