Similar to "brother from another mother", "fam from another clam" indicates that an individual is very close or tight to another person, despite the fact that they are not necessarily blood (ie. family born from another mother). This tie to an individual is essentially the tightest relationship possible without being family.
"Yo, Kenny! Mah fam from another clam! Wassup?"
"Man, this guy's stuff is dope, trust me. He's my fam from another clam."