It's probably a typo. Probably...
In Strange Tales 119 Dr. Strange needs to enter the Purple Veil to save two crooks who tried to steal a gem that was the doorway to that dimension. So he calls his Mormammu to help him
This blog about Strange Tales 118 suggests it's a typo
When he said Mormammu, I think he meant Dormammu. In fact, the Marvel Masterworks reprinting changes the text to read “Dormammu”. “Mormammu” was perhaps a typo. Or maybe Dr. Strange messed up the invocation, which probably would anger Dormammu.
And again about Strange Tales 119
One confusing thing about the invocations is Mormammu vs. Dormammu. In this comic, he refers to both. The Marvel Masterworks I am reading changes all instances of “Mormammu” to “Dormmamu”, so I am sometimes uncertain which was used originally.
I have two other reprints of this story, which disagree with each other. Giant-Size Defenders #2 kept the first reference as Mormammu. Marvel Collectors’ Item Classics #9 changed the first reference Dormammu. The second reference is definitely to Dormammu. In order, the below pictures are from the original comic, Marvel Collectors’ Item Classics #9 (1967), and Giant-Size Defenders #2 (1974).
And indeed, they have this version from a later reprint (panel #3 from the previous strip) that has it as Dormammu. Yet another later printing after that went back to Mormammu
So... is it a typo, or was it intentional? The original Stan Lee work has it as Mormammu. Someone at Marvel thought it was a typo and "fixed" it, but someone else put it back to the original. Since this is the only time it's ever really used, it strongly suggests a typo, but the story itself about the Purple Dimension is a one-off and Strange does not go there again.