When an opening letter-name contains madd followed by a still consonant which assimilates into the beginning of the next letter-name, producing doubled articulation, the rule is madd lazim harfi muthaqqal. It combines obligatory elongation with the subsequent doubled consonant.
In «الم», after pronouncing “alif” and then “lam”, the final mim of the name “lam” joins the initial mim of the name “mim”. Assimilation creates a reinforced mim sound. The madd within the name lam must still be given its proper duration.
This does not convert the separate opening letters into one ordinary word. Each letter retains its name while their sounds interact during connected recitation. Keep the long vowel, lip articulation and doubling all audible in the correct order.