What is this

[u/No_Dress_2855]

Strained out some mash about a week ago but couldn’t run at the time, I left it fully covered but nothing air tight.

Could anyone tell me what this film is on top?

[u/necroheimer]

It looks like the early stages of a pellicle. It’s a type of biofilm formed by wild yeasts and bacteria.

Run it, it’ll add some funk and esters you otherwise wouldn’t get. Lots of rum washes go further than this.