Is using "more correct" a grammatical faux pas?

I caught myself saying "to be more correct", is this strictly possible given that something is usually correct or incorrect. If this is a grammatical faux pas, what is it called?

"More correct" is acceptable (especially in the adverbial form "More correctly"). That said, you will usually see "More accurate" instead.

"More correct" is certainly used when talking of forms of address. For instance it is quite common to call a Church of England parish priest "Reverend" when speaking to them but Debrett's {if you follow the link scroll down to the table of forms of address at the end} would not recognise this as the correct form of address (it is correct in the US but not in the UK).

In general it is more correct to use "Vicar" but, sometimes, a parish priest is a "rector" rather than a "vicar" and calling a rector "Vicar" is not really correct (although you can't tell the difference if you haven't been told). It is also possible that a rector or vicar is also a canon or a dean (or even a retired bishop) and in those cases "Rector" or "Vicar" would not be the truly correct form of address, however the relevant one would still be more correct than "Reverend".

Fortunately people worry about such things less now than they used to do but the distinction between correct and more correct forms of address still exists.

'More correct' is semantically questionable and you probably want to word it differently.

Adjectives can be described as 'gradable' or 'absolute'.. A gradable adjective has degrees of that quality. An absolute adjective either has the quality or does not, with no in between.

The color 'red' is a gradable adjective because the redness of an object can be placed on a scale. This one is red while this one is redder. The semantics of the adjective allow comparative and superlative syntactic constructions easily.

Another example of an absolute adjective is 'unique'. If there is only one of a particular kind, it is unique. It cannot be 'more unique' - it is already unique, and that's as far as it goes.

Absolute adjectives do not allow being made into comparative or superlative. You can say 'the final countdown' but not 'the most final countdown'.

There are not many absolute adjectives. As the saying goes, you can't be a little pregnant, you either are or are not. But within a set of pregnant women, some may be further along than others and one can very legitimately say that one is more pregnant than another. So many adjectives that seem logically absolute, allow a little grading in comparison.

Which brings us to 'correct'. We are all familiar with the idea that true/false or multiple choice questions on tests are either correct or not; there's only one right answer. But one can always make a thoughtful philosophical point in a particular situation. "This answer is correct, but this other answer is better - it is more correct.". That might work in some situations, but the situation you'd really have to explain yourself to make sure you're not committing a solecism. Making a comparative out of an absolute usually sounds bad and is a sign of not thinking things through.

The adjective 'correct' is usually absolute, but there are situations where it can be thought of as gradable.

As to what it is called when one uses a word incorrectly or, more charitably, in a nearby sense, that is:

catachresis.

The rhetorical term for a grammatical error is

solecism.