If it's empty then fill it up with a vinegar/water mixture and add a powerhead to circulate the water. Let it sit like that for a day or two and you should be able to scrub off all the buildup with a sponge. The wavemaker in my reef tank became so encrusted in coralline algae that it was losing power and a vinegar soak allowed me to wipe everything off and make it look brand new again.