use a oil based solvent that removes wax from molds etc. Being an oil based solvent is may also stain your patio as well so it would be good to test it in a piece of scrap if available...To remove the "solvent" of the wax remover a liquid dish soap will do a good job..this and lots of rinsing scrubbing with a scrub brush it should bring it back to near normal... once you get it cleaned up, seal everything to reduce this from happening in the future.

it is you decide to use. Only tape or in the ABS pipe you can use glue. Just be sure to install a clean out Y fitting somewhere where you can run a air hose or vacuum down the pipe. Also Handy is right on about insulation if the pipe is located in a cooler environment where it can condense not only on the outside causing mold and damage, but inside also as the air cools down on way out the moisture condenses and turns to water in the pipe which will cause the lint to stick.

The last concern I have is the length of the pipe. Check with dryer install about max length. Each 90 degree turn can in effect add around six feet of length to the overall run size. So if your turning down then across your 25 foot run is now 32 feet overall. Not suggested for safe and efficient operation of the dryer.