Aw, you took the character out of that bird bath. What will the birds think. Mother Nature abhors a vacuum, how long do you think it will take her to create life in that sterile environment? What is that stuff? Sediment, stain, and microbial life. A world of its own.

Don't worry, it won't be that long before Mother Nature reclaims it.
A cleaner? I usually just use a brush though that reminds me, I need to clean some algae off of a few walls and a shed door. I usually use a bleach solution and elbow grease.