You can convert, something like
julia> v = rand(3,3);
julia> C = findall(x->x>0.5, v)
5-element Array{CartesianIndex{2},1}:
CartesianIndex(2, 1)
CartesianIndex(2, 2)
CartesianIndex(3, 2)
CartesianIndex(2, 3)
CartesianIndex(3, 3)
julia> L = LinearIndices(v)
3×3 LinearIndices{2,Tuple{Base.OneTo{Int64},Base.OneTo{Int64}}}:
1 4 7
2 5 8
3 6 9
julia> L[C]
5-element Array{Int64,1}:
2
5
6
8
9