A 60-year-old man with type 2 diabetes mellitus has a routine urine test showing an albumin:creatinine ratio (ACR) of 5 mg/mmol (normal <3). His blood pressure is well controlled at 125/75 mmHg on ramipril 10mg daily. His eGFR is 70 mL/min/1.73m². According to NICE NG203, what is the significance of this ACR result?