Slim Build Up in Water Reservoir

[byee]

I've been using a 5 gallon salt bucket as a RODI water reservoir for my ATO. I've also tossed in an old maxijet 400 to run for about 15 minutes every few hours to keep the RODI water circulated

I've noticed a slim build up after a few months at the bottom of the bucket as well as the maxijet 400.

Does anyone know what and why there is a slim build up? Anyone else noticed the same in there RODI water reservoirs?

Thoughts or opinions?