Get an Intex pool cover and some algaecide. Clean your pool; add a double dose of algaecide; run the filter for 1 hours with the cartridge removed, and then cover.

If possible use polyquat: http://www.poolsolutions.com/polyquat.html ; if not use regular foamy algaecide with NO copper. You'll have a bit of sudsy mess when you return, but it's much better than algae.

You may not be able to buy an 18' cover -- if not, buy a 20 x 20 tarp at Lowes and some 1/8" nylon rope, and tie the tarp in place over the pool.