The grammar is best understood as containing an ellipsed verb "be" (with subject pronoun, of course). The complete clause is "If it is left to stand".
So the tense is present tense, passive voice. To clarify:
"If one leaves it to stand, the cooked pasta sticks together." (ACTIVE VOICE)
"If it is left to stand, the cooked pasta sticks together."
The passive voice here combines present-tense be with the past participle of "leave", namely, "left". Don't be misled by the fact that "left" is also the past simple of "leave".
I worked precisely the same grammar into my first sentence in this response (present tense, passive voice). See it?
The combination of two present tenses in a conditional structure like this is called Zero Conditional.
There is another way of reading the sentence that makes sense to me:
"If it has been left to stand, cooked pasta sticks together."
(In active voice: "If one has left it to stand, cooked pasta sticks together.")
The difference in meaning is subtle and neither one is clearly suggested by the original, which omits the verb be ("is"/"has been").