Efflorescence. Minerals (calcium, salt, lime) being leached/washed out of the concrete due to moisture such as a water leak.
Is it possible that what ever plumbing work you had done was to correct a water leak or perhaps created a water leak?
there's a new furnace installed - so as far as we know this access door isn't connected to anything inside the house. It only recently start leaking that crusty orange stuff so water must have gotten in there somehow.
We googled chimney cleanouts and the pictures we found are exactly like what we're dealing with. Apparently all the raining recently caused the flooding. I need to get the chimney cap fixed and have something brought in to seal the cleanout permanently.