While the owners may not be a marijuana related business, the funds are from an MRB, and therefore derived from the sale of cannabis. The good news is that the funds are coming to you as checks, and not cash, which means their MRB is banked.
It is likely that the bank that the check is drawn on is doing to the work to validate they are licensed, not showing any red flag behavior, and doing the SAR filing. You might want to reach out to them to see if they have a strong cannabis banking program and that would potentially give you some comfort that the funds are from state legal sales.
With that said, you might want to consider raising your risk rating on them as customers since they are so closely tied to the industry, and making some notes. That way if anything suspicious comes through monitoring you'll have a head start on working the case.