The crab will not be bothered by the algae and depending on the species of spider may have actually placed it there itself. The algae itself will not harm the tank really unless corals are in danger of getting smothered. If/once the sources that fuel the algaes growth (mainly PO4
) are eliminated, the algae will die anyway.
I read in a previous post your PO4
was 5? Very, very high. It will either be from foods, the water source or both. When using granular media to remove the PO4
, it needs to be changes out every 24-48 hrs until it gets below 0.5 ppm
. Then you can either change weekly or longer depending on what brand of media you use and the PO4
levels at the time.