this saying has its origins in the 70s love movement and was made really famous by Cheech & Chong in the 80s through their marijuanha films.
It does not mean OMG or awesome. It's a stand-alone phrase, usually preceding the phrase that will describe the object or situation that's freaking you out, blowing you away or leaving you at a loss for words.
It's present tense, denoting something that's actually happening and feels like it is beyond incredible.
Holy mother of pearl! Look at those tender young asses(on the beach);- cops in helicopter shadowing Cheech & Chong.
Holy mother of pearl! That's some serious shit.
Holy mother of pearl! That's one strong fuckin drink.
Holy mother of pearl! Did you see that?(a flying dog playing a guitar and shooting laserbeams from its eyes)
A spaceship appearing over London would leave you speechless, probably only with the ability to utter the words 'holy mother of pearl.'