Typically with an ESPP, you will already own the stock at passing and it will be held in a brokerage account. Any stock not actually purchased (owned) yet will not be distributed to any beneficiary.
ESPP stock in your brokerage account is treated as any other stock you own. If you pass, the beneficiary will received a stepped-up basis. This is usually the value on the date of your passing.
Note: Some estates will chose an alternate valuation date (usually 6 months later) but this is rare.