Probably Turf Snow Mould, or fusarium patch disease, a fungal infection. Occurs in particular on poorly aerated turf, and on areas which have been walked on when snow cover is present. Needs treating with a suitable fungicide (not sure what's available now, nor when its best to use it, check products either on google for this infection, or the garden centre, and use as suggested) and improve the aeration by spiking regularly, and don't use high nitrogen feed on your grass past mid July at the latest.

I am not sure that it does need treating, Paper and Bamboo, at least not just yet. This is the norm in the mountains after the snow has gone back. The top grass has been denied light due to the snow blanket but greens up again fairly quickly as the snow goes back. If it is still white later in March when it starts growing again then, maybe, you should start treating it.

Good point, bulbaholic - but you'll know, because if it is fusarium patch, the white patches will go brown and the grass along with it, dying as it turns.