There is a dry white film on the surface of my milk kefir ( the kefir grains still inside )after half to 1 day of brewing depends on temperature.

Does any one know what is this dry white film? I

s it some kind of mold & is it harmful to consume it?

At the moment, I try to skim off this dry film before straining the kefir, but I don't think I can completely skim off this film & I worry about consuming this film if it's some kind of mold.

What causes this dry film & how can I do to prevent this dry film happening?

My kefir grains seems to be less active after I fridged it with more milk for 4 days while away, will it make the grains less healthy?

Thanks for any advice!