Short answer: No. A properly structured group medical stop-loss captive is not a MEWA. Each participating employer keeps its own single-employer self-funded health plan, and the captive insures the employers through stop-loss, not the employees, so no single plan covers the workers of two or more unrelated employers. An Association Health Plan, by contrast, is a type of MEWA.
A MEWA (Multiple Employer Welfare Arrangement) provides health or welfare benefits to the employees of two or more unrelated employers. A captive is an insurance company owned by the businesses it insures. The two are often discussed together because both are ways for employers to pool and take on risk, but legally they are different things.
In a properly structured group medical stop-loss captive, each employer keeps its own single-employer self-funded health plan. The captive provides stop-loss insurance to the employer, which protects the employer against unusually large claims and is treated as insurance for the employer rather than health coverage for employees. Because the risk pooling happens at the stop-loss layer and not at the level of an employee health plan, there is no single plan covering the employees of two or more unrelated employers, and therefore no MEWA.
The line can blur. If an arrangement is set up so that employees of multiple unrelated employers are actually covered under one shared plan, regulators can treat it as a MEWA. That matters because self-funded MEWAs are subject to state insurance regulation (licensing, reserves, and examinations) on top of federal ERISA rules, and MEWAs have a history of failures and unpaid claims. A captive that stays within the single-employer-plan model does not become a MEWA, but employers should have benefits counsel confirm how any specific arrangement is structured.