The fungus is growing on the rotting berries. He describes the gray (he uses the British spelling, "grey") fuzz like a rat stuffing himself on their stash.
A cache is a collection of items stored away. So, again, we're reminded that this is their secret stash.
Heaney sure isn't afraid of the gross. Though the real image is berries in a tub that are a little moldy, he compares it with an image of a rat stuffing himself with the berries. The two images are mixed in our heads while we read, for a more exaggerated effect.
The juice was stinking too. Once off the bush The fruit fermented, the sweet flesh would turn sour.
The juice of these berries smelled. Then he tells us that once the berries are picked, there's no way of keeping them – they immediately start to rot.
He uses "fermented" to describe how the berries sour, which is accurate. It's also the natural process that juice undergoes to make wine. This is not the first mention of wine in this poem! Remember the Eucharist reference?
Also, he uses "flesh" again.
He seems to be bummed about doing all this work, picking as many berries as possible, only to have them spoil.