What in the world is E Hero Neptune? A google search did not return me any result.
But anyway I am just gonna assume that it is a Water Attribute E hero monster with no effect since you did not state any.
In this case, the chain is
Chain 1: Bottomless Traphole
Chain 2: Mask Change
Resolving the chain
Mask Change summons Acid (at this point you cannot activate acid effect because bottomless is still on the Chain)
Because Acid effect is mandatory, it is now put on a new chain which your opponent can respond to which in your case is Infernity Barrier.