Praise be to Allah.
It is obligatory to remove anything that may prevent water from reaching the skin so that one may certain that the part in question has been washed as Allah, may He be exalted, enjoined. Some scholars overlook that which is difficult to remove, such as dirt under the nails or in the cracks on the feet.
It says in Mataalib Ooli an-Nuha (1/116): A small amount of dirt under the nails and the like does not matter, such as inside the nose, even if it prevents water reaching the skin, because that is something that usually happens frequently, and if wudoo’ were not valid in that case, then the Prophet (blessings and peace of Allah be upon him) would have pointed it out, because it is not possible that he could have delayed explaining something at the time when people needed to know it.
Shaykh Taqiy ad-Deen included with a small amount of dirt every small thing that could prevent water from reaching the skin, such as dried blood or dough on any part of the body. He favoured this view by analogy with what may be under the nails, and included in that cracks which appear on some parts of the body. End quote.
It says in Haashiyat ar-Rawd (1/204): … such as something inside the nose, which prevents water reaching the skin; his wudoo’ is valid. This view was favoured by al-Muwaffaq and others, and was described as sound in al-Insaaf. The shaykh said: Any small amount of dirt on any part of the body, and the dirt that may gather in the cracks on the feet is overlooked, and he included with that any small thing that prevents water from reaching the skin, wherever it may be on the body, such as dried blood, dough and the like; and he favoured this view. End quote.
So the dirt that appears in the cracks on the feet may be overlooked, assuming that it is there, because it may be skin that has changed colour and not dirt that prevents the water from reaching the skin.
Based on that, praying behind this imam is valid.
And Allah knows best.