First..do a moisture test on your existing slab. You want to see if moisture vapor is coming up into the basement from under the slab. Tape a 2' square sheet of poly on the floor and see if there is any moisture under it after 48 hours. If the moisture is significant, it might play into what type of membrane you use on the floor. Memebranes like Hydroban and Kerdi have similar properties when it comes to liquid moisture, yet differing properties when it comes to moisture vapor transmission. Significant moisture drive could actually lift and delaminate a topical membrane from the slab. If you have under-slab drainage or radon ventilation pipes, and foundation perimeter footing drains, you should be in fine shape. I'd make it a wet room, pretty much follow Bill's advice; -slope the entire floor to the drain. -topical membrane on the entire floor and up about 6" on all the walls, or just fully membrane all the walls. Fully membrane the shower walls. -ventilation. Don't skimp. This room will be cool and the basement itself is typically humid, so latent drying action will be less than in an upstairs bathroom. Do what you can to eliminate moisture vapor as it is generated. -I'd not insulate or heat the floor. You might have to balance relative humidity in the basement as a whole with a heat source in the bathroom. If you do have vapor drive into your basement, it might help to have something as simple as an IR bulb on a timer to help with the initial drying of the room. For a humid, slow-to-dry shower environment; A non-porous porcelain tile might be better than a porous natural stone tile. An epoxy type of grout might be better than a portland-based grout.

I have that same problem with my shower. The floor is 4x4 white tiles, though. I even had it re-grouted, let it dry out, and the mildew/mold came right back, even though I dried the shower after every use with a towel. While doing that, I noticed that water was seeping back up through the grout after I dried everything off. My plumber told me something similar to what lazygardens said, the slope is not right, and it needs to be torn out and re-done. You might want to have a plumber look at it. I'm planning to go ahead and re-tile the whole bathroom in addition to putting in a new shower, and do a little more updating, so I'm in the process of trying to find a contractor and figure out what I want. Torn between a new tile shower or some kind of solid surface so I won't have to deal with moldy grout ever again.

I'll give a slight nutshell answer or analysis...first, you have an older home and you sealed the "lid" by spray foaming the attic. Was anything done to the basement in terms of moisture mitigation? Old houses tend admit moisture in the house through the basement. Could be a rubble foundation, moisture coming through a poured foundation, etc. It might not always be liquid, it can be simple wicking and moisture vapor. Before your house was tightened up, this moisture vapor likely passed through your house due to excessive (by today's standards) air infiltration; leaky windows, inadequate insulation, an open attic, and plumbing, electrical, and mechanical penetrations in the varying floor platforms, etc. It could be that you still have the same moisture coming in to the house through the foundation and basement or crawl space, but it's getting trapped on the higher floors due to the now tightened attic. Moisture generation points might need to be looked at too. Kitchen ventilation, bathroom and shower ventilation, etc. If moisture coming on through the basement walls is an issue, that should be addressed. If you have penetrations through the basement ceiling into the living space above, those can be sealed with canned foam. Now, you sealed up the attic in terms of the attic no longer being vented, but is it now part of the conditioned space? ie, are there any air conditioning vents or returns up there so air is circulated and conditioned? Or is it just dead space?

Windows in a shower can be very problematic. Perhaps you should leave the window as it is and add an exhaust fan.