The only valid answer here has to be 'read the policy document'.
For many years, almost all insurance policies (whether comprehensive or just third party) have covered the insured person (at 'third party' level only) for driving any other vehicle as long as the vehicle is not owned by by the insured person or hired by him/her. (Some policies have had additional restrictions about driving vehicles owned by a family member, especially if living at the same address).
However, in the race to offer the lowest premiums, some companies now exclude this 'third party/any vehicle' provision from their cover.
If you own the moped, you're almost certainly not insured to drive it on your car policy (unless, of course, you've specifically had it added to your policy). If you don't own the moped, you might be covered but, as I've stated, you need to read the policy document to be sure.
Chris