Pumpkin mousse looks grainy and slightly watery
No, you cannot turn this mixture into a mousse. You have to eat it as it is - which is probably simply a tasty creamy pudding without the expected mousselike consistency. Even if it is grainy, there is no reason not to eat it that way. Unless you find it utterly impalatable, in which case you have to throw it away. It is normal to expect some failures with such an intricate recipe, especially if you have little previous experience with patisserie.