All perfect praise be to Allah the Lord of the Worlds. May His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
The general rule is that when a Muslim stands in prayer before Allah, he/she must stand in a manner of humility and submission to Him, keeping far away from everything that may corrupt or invalidate his/her prayer. He/she must make sure that his/her mouth is clean prior to commencing prayer. However, if he/she puts a piece of candy in their mouth intentionally although aware that it is forbidden to eat or drink while praying, and that piece of candy was of the melting type, then his/her prayer is invalidated if it melted, but if it didn`t then his/her prayer is valid.
"If one had a piece of candy in his/her mouth when praying and it melted and he/she swallowed it deliberately-knowing that this is forbidden or was negligent to learn this ruling-then his/her prayer is invalidated, and this is in accordance with the most correct opinion of the Shafie school of thought." {Nehayat Al-Mohtaj Ala Shareh Al-Minhaj, Vol.2:p.52}.
However, if the piece of candy was there forgetfully and the praying person didn`t remember that throughout the prayer or was negligent of the prohibition-as one who is a new Muslim and didn`t know the rules of prayer-and swallowed a small amount of it that had mixed with his/her saliva, then his/her prayer isn`t invalidated. Nonetheless, if he/she swallowed a large amount then his/her prayer is invalidated. This is because this act is similar to eating food intentionally; this act renders prayer invalidated. Imam Al-Shirbini said: "If there is a little portion of food in the praying person`s mouth, then the prayer is invalidated because this act contradicts prayer itself. This is unless the praying person was forgetful or didn`t know the ruling because of being a new Muslim or nobody taught him/her the ruling, in which case little food definitely doesn`t invalidate prayer, Allah knows best, because it doesn`t contradict it." {Moghni Al-Mohtaj, Vol.1:p.419}.
In case the person was praying behind an Imam, then the above rulings apply as well. However, if the Imam himself had that piece of candy in his mouth deliberately and knowing that this is forbidden then his prayer is invalidated and he must repeat it.
It is worth pointing that if the person led in prayer knew that the prayer of the Imam is invalid, such that the latter had a piece of melting candy in his mouth, then he must leave the Imam immediately or else his prayer is invalidated as well. If he didn`t know, then his prayer is valid because this is amongst the hidden nullifiers that the persons praying behind the Imam are unaware of. And Allah the Almighty knows best.