Table 23 "Functionalities depending on the working mode" of the STM32L4x6 reference Manual (my copy is Rev 6) suggests that you can't turn it off.
They say you can gate the clock in sleep mode with bit 9 of RCC_AHB1SMENR.
But the corresponding bit in RCC_AHB1ENR is marked as "Reserved, must be kept at reset value."
I see that the current consumption of SRAM1 is 2.29 - 2.80 µA / MHz, so clocking more slowly will reduce its current.
But you might find the best approach is to get processing done quickly and have the processor sleep until there's enough new stuff to process.
Hope this helps,
Danish
