X y parallax of the layer must be to x 0 y 0 if u deisable only y that means parallax will still happen on X axes just wont happen on Y ... so you need to disable both axes for it to work.
if you cant manage it ... set the position of the hud to ViewportLeft(0)+300 ViewportTop(0)
layer its the layer,(can take names "layer1" or direct value of your layer (from 0=the bottom position top any number you have that is greater then other layers you might have above)), you want doesn't matter the viewport applies to all anyway... its not like you have 2 viewports.