Frost me, no, they are not the same thing. (I own a number of Majorica necklaces.)
Majorica imitation pearls (note, this is a brand name, as opposed to Mallorca, Majorca, Joia de Majorca and other competing brands) are made with an opalescent glass bead inside. This bead is dipped many times (up to 40) into the type of lacquer traditionally used to make imitation pearls. I know they derive substances from fish scales, for example. I understand they polish between coats. The final coat has something that makes it appear as through there is orient.
There is strict quality control with Majorica-- you won't find defects in the pearls. They are very durable and I have not found them to discolor over time. Of course they can be damaged by exposure to the usual offenders (perfume, hair spray etc.) but they are more resistant than genuine nacre is. All in all they are excellent imitations.
Shell pearls may have a shell bead inside (which may be ground-up reconstituted shell material) and the coating is made from ground-up mother of pearl, which is iridescent. This is how they justify representing the pearls as "mother of pearl" when, in fact, it is just a coating. The shell pearls I've seen are on the larger side, to imitate SSP and Tahitians.
I think imitations have their place but the seller has to be very clear with customers about what they are.