The substance that looks rather like ice in this photo, which was taken on a warm and humid day, is the slime mould Ceratiomyxa fruticulosa (var. fruticulosa).
More precisely, this is the plasmodial (feeding) stage, in which the slime mould is in the form of a plasmodium: a mass of protoplasm contained within a cell membrane (rather like a giant amoeba, and moving in the same manner, namely, protoplasmic streaming; however, a plasmodium, unlike an amoeba, contains multiple cell nuclei).
The plasmodium shown here was transparent and gelatinous, but the photo also shows traces of the delicate silvery patterns that were evident within; these perhaps indicate the direction of the streaming flow of the protoplasm. Ceratiomyxa fruticulosa has a plasmodium that is more massive than that of most other slime moulds (plasmodia of the species Brefeldia maxima can also be very large).
In this case, the species could be identified because an example of the next stage of its development, the spore-bearing stage, occurred immediately next to it. Not far away, on the same day, I encountered a very extensive example of that stage of the life-cycle: NS3984 : A slime mould - Ceratiomyxa fruticulosa
The spore-producing stage of this species is often encountered, but its plasmodial stage is not quite so familiar. Of this species, the textbook "Introductory Mycology" (2002; Alexopoulos, Mims, and Blackwell) says: "In spite of many persistent attempts, C. fruticulosa has not been grown in laboratory culture through its entire life cycle and plasmodium formation has not been observed. In nature the plasmodium has been observed only as a mature structure just before it is ready to sporulate".