If pH is 7.8 and total alkalinity is 240 and the 12'x3' pool is starting to get green stuff in the bottom what should you do?

Add muriatic acid to lower your TA to a more acceptable level...around 100, but no higher than 140. Then balance with bicarbonate of soda if necessary to increase ph. Once you get to an acceptable TA add an algaecide or shock the pool to get rid of the algae on the bottom.

Just triple shock with liquid chlorine; for your pool 2.25L. You probably need to add water (because of splashing) quite regularily so I wouldn't bother lowering the alkalinity.