Is there a word for the material of a forest floor?

There is mast which refers to beech nut husks, acorns etc. Lexico has

mast²

NOUN
The fruit of beech, oak, chestnut, and other forest trees, especially as food for pigs.

Here is a typical picture of beech mast.

_{Alamy stock photo}

That's also called duff.

Duff: the partly decayed organic matter on the forest floor. [M-W]

Or humus, perhaps?

Humus: Dark earth made of organic material such as decayed leaves and plants.

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Detritus could also be used but it's broader than duff and leaf litter.

The term detritus is broadly defined as any form of non-living organic matter, including different types of plant tissues (leaf litter, dead wood, aquatic macrophytes, algae) animal tissues etc. [Detritus and decomposition in ecosystem]

Where there isn't soil in the forests of the Pacific Northwest it's called muskeg

Muskeg (noun) A North American swamp or bog consisting of a mixture of water and partly dead vegetation, frequently covered by a layer of sphagnum or other mosses.

The part of the forest floor that is composed of a decomposing compost of leaves is historically known (at least in England) as leaf mould, which, since it begins with an L, is most likely the term you were half-remembering.

In general, though, unless you're specifically talking about the rotting stuff, go with forest floor, or @decapitated-soul's suggestion of detritus.