Does melanostatin peptide whiten the skin in a similar manner as to how melanotan darkens it?
Melanotan

The "Melanotan" you're referring to is probably Melanotan I, also known as afamelanotide, which is an analogue of α-melanocyte stimulating hormone (α-MSH).  In other words, it activates (some of) the same receptors as α-MSH.

α-MSH has several functions, one of which is to stimulate the cells that darken the skin (melanocytes), so they produce more melatonin.  It activates these cells via the MC1 receptor.  Afamelanotide is more specific to that receptor, so it darkens the skin without some of the other effects.

It acts at the hormone level, so it won't be changing anyone's genetic makeup - stop using it and you'll go back to your normal colour.

Melanostatin

Confusingly, the name Melanostatin has been applied to more than one compound.  I've found two short peptides that might fit the bill.

The most likely candidate is described in this leaflet, which says it is used for skin lightening.  It's a receptor antagonist - it prevents α-MSH from binding to the MC1 receptor, so reducing melanin production.  So yes, it's working at the same site as Melanotan I but having the opposite effect.

The other possibility is also known as Melanocyte-inhibiting factor, which inhibits production of (natural) α-MSH, so it would be expected to lighten the skin.  However, since α-MSH has several other functions, reducing its production would also affect other systems - there would be side-effects, and skin lightening doesn't seem to be the main thing it's known for.  More here: Wikepedia