For newbies reading this thread, it is important to understand that rain, unless it washes water INTO your pool from the surrounding area, should have little affect on pool water chemistry.
The vast majority of pools are simply unaffected or influenced by rain, even a pretty heavy one.
In a 48" deep pool (average) a 2" rainfall adds 4% new water so, even though this is an oversimplification, changes your chemistry by roughly 4%
Algae on the bottom of a pool is not caused by rainwater but by lack of chlorination prior to seeing visible algae.
Blackbeard, if you have clear water and algae on the bottom of your pool, you should be vacuuming and brushing your pool to remove as much as you can and then you should begin the SLAM process.
read "The ABC's of Pool Water Chemistry" up in Pool School
read "SLAM - Shock Level And Maintain" in the How To section of Pool School