AG pump size is very hard to gauge based upon just the motor label. But a 1.5 HP SPL is the equivalent of a 1 HP up rated pump which is equivalent to a 3/4 HP full rated pump. Add to that the fact that AG pumps usually have lower head curves, your pump is probably not all that big for that size filter so I would say you are probably ok.

The issue you are seeing could be due to either the filter or the debris you are trying to filter. Most sand filters are suppose capture particles greater than 20 microns but some algae is smaller than that and can travel through the filter. One thing you could try is allowing the filter pressure to rise more before cleaning as that will help to capture smaller debris. Also, I think some people have successfully used DE in sand filters to help with debris capture but I don't have any experience with that.