This belongs in plant problems.
What does it look like? It is white and crusty on the hydroton? Is it brown and slimy on the roots? What is the ambient humidity in your grow area? Description would help.

Cannazyme is the enzyme product that helps break down dead cells into their basic components and allow re-uptake as nutes by the plant. Use it CAREFULLY, since the break down process will release a bunch of stuff into your res. I've killed a mother with the stuff; she ended up dying of a SERIOUS and sudden nute overdose... so when in doubt, flush!!!

I'd do a hefty rinse and a res change, run with peroxide and cannazyme for a day, and then change the res again back to your normal nute solution.

I've never dealt with a serious slime buildup though so take my advice as a hunch more than experience.

I find that my brown res slime/sludge just gets stuck on the pump filter so I rinse that off weekly and remove any other solids from the res with a small strainer.