Ay mama, totally get your concern, mucus sa poop is really nakakaworry kahit alam natin normal lang siya sa breastfed babies! Since your baby is happy, alert, at walang blood, it's likely just that, pero I'd really suggest checking with your pediatrician next visit just to be sure, lalo na na nag-introduce kayo ng formula kaya good idea i-rule out kung may lactose sensitivity or formula adjustment na kailangan.
Most of the moms I know na may same experience, reassured lang sila ng doc na normal talaga, especially if baby's gaining weight at content, kaya wag mag-worry masyado, you're doing great mama!