Odd build-up in my container...

[MT-Phal]

Recently I've noticed a strange build up of white, jelly-like stuff in my s/h container. When I water, it will float to the top and then come to rest on the top layer of PA. It washes right off... but does anyone know what this is? I always flush with warm water before I use my fertilizer solution. I've photographed an example of some build up by my resevoir...