If I remember my chemistry the gas you're seeing is water vapor in the air hitting the surface of the acid and fuming. Muriatic is hydrochloric acid at a high concentration. Any time you add water to acid the reaction gives of heat and in the case of a high acid concentration adding water could cause the solution to boil violently so you never do that. Always add acid to a large quantity of water as you are doing. as Dave said, just run at a ph of 7.8. All other reading being 'fine' says nothing. Get a decent test kit if you don't have one and post some actual numbers. Your pool guy is wrong to say not to add chlorine until the ph drops. You need to do it ASAP. Again, to give more accurate advice we need actual numbers.
FC or TC -- Free or total chlorine
ph
TA -- total alkalinity
CH -- calcium hardness
CYA -- cyanuric acid
While you're at it, what type of pool?
Al