Excess nutrients, no native plants, and the OP just moved in last year. Any time a pond goes months or even years w/o maintenace, it can get overrun. OP is working on getting it back to full glory....
If yor parents just finished their pond, and it is not full, the nusiance plants and algae shouldn't be a problem this year. Snow melt will probably fill it in depending on size. Pond dye should probably be used next spring until some desirable plants fill in and take hold.