That slow video clip shows two chunks of white stuff falling together. One misses the wing OK, but the other hits underneath and is smashed to fragments, but they BOUNCE away from the underwing, implying that they are solid and have momentum.

Ice looks more likely than foam.

Also, the design problem(s) that ultimately led to this accident are:

1) Having wings in the first place (An air force requirement, now rendered null by the air force abandoning the shuttle)

2) Having the shuttle mounted below and behind the ET, not on top of the stack like all other rockets (A feature resulting from the desire to have reusable main engines in the Shuttle itself)