I have acquired control of a small ranch house (don't ask, it just "happened"). Small, cheap house on a block foundation in Zone 4/5 very humid microclimate. House is about 30 years old. Has mostly a dirt floor crawlspace with a small area with 6' headroom near bulkhead where the boiler and water heater reside. Crawlspace is unconditioned except for the heat from the near-boiler piping (mostly uninsulated. Block walls not insulated. Wood floor above was insulated with kraft faced batts with kraft down. Whole space was loaded with mold so insulation was stripped out mold sprayed down and dirt floor covered with poly. There is no access from above - just and exterior bulkhead. Question: What is the best treatment for the floor deck? Spray foam with closed cell? Spray interior of block walls while there? Is turning the poly up the wall and sprayfoaming the junction of wall and poly help seal the poly to the wall? I feel any sort of batt (FG or Rockwool) will just grow more mold. I'd love to save some energy but the primary focus now is moisture and mold control. Thoughts??

Honestly, if moisture control is a consideration, maybe look at pouring a thin rat-slab over the dirt floor.
Possibly add a french drain around the interior and exterior of the foundation before the pour.

is the crawlspace ventilated?

If you take moisture mitigation steps and ventilate properly, your choice of insulation should be irrelevant.

MOST ideal would be to turn the crawlspace into conditioned space.
But you may not want to go to the expense.

Is there a boiler down there? If yes, insulating the subfloor will isolate the standby and distribution loss heat from the space above, increasing the net heat load, without doing a whole lot for moisture control.

Air sealing and insulating the block walls to IRC code-minimum (R10 continuous insulation if zone 4, R15 if zone 5) and ventilating the crawlspace to the code minimum 1cfm for every 50 square feet of floor area will control mold. Ventilating it in a loop with conditioned space air from upstairs is fine, as is exhaust-only ventilation. If you haven't already, seal the vapor barrier to the CMU walls about a foot up from the floor with a bead of polyurethane caulk between the plastic & wall, with the edge of the plastic sealed to the CMU with duct mastic. If you can fit 4x8 sheet goods through the access hatch it's pretty easy to insulate the walls with rigid foam. Fire rated Dow Thermax can be installed without any thermal barriers against ignition, but cheap reclaimed roofing polyiso with a layer of (unpainted) half-inch wallboard works too. Tape any seams between sheets for air tightness, can-foam or caulk the seams to the wall at the top & bottom to prevent convective moisture transfer. In many locations mounting the foam with just foam board construction adhesive is good enough, in others it may require 1x4 strapping through screwed to the foundation with masonry screws. (The strapping becomes a convenient nailer for hanging gypsum board, if you go that route.)

If you can't get sheet goods down there, 2" of closed cell foam (preferably HFO-blown goods, which runs about R7/inch and has very low impact blowing agent) painted with an intumescent paint for fire control flies in most jurisdictions, but ask the inspectors/code-enforcers first.

If the walls are insulated you can leave the batts in the floor or take them out- the effect on energy use is negligible, and it will make the temperature of the crawlspace closer to the fully conditioned space above, further minimizing condensation risk from air transported moisture from ventilating it with conditioned space air. Kraft facers exposed to the crawl space are their own fire hazard, and would be a code violation in my area if there was an ignition source (such as a boiler) in the crawlspace.