Wastewater treatment plants act as microplastic (MPs) sinks and secondary MP pollution sources. Little is known about the effect of MPs on biomass and the efficiency of biological wastewater treatment. This study assessed the impact of polyethylene (PE) MPs concentrations (1, 10, 50 mg/L) in wastewater on biological conversions and extracellular polymeric substances (EPS) production (including alginate) in aerobic granular sludge (AGS). PE MPs did not worsen the efficiency of biological treatment but stimulated the production of EPS and alginate in AGS. The alginate content increased from 238.7 ± 4.4 mg/g MLSS in control to 441.6 ± 13.8 mg/g MLSS at the highest PE load in wastewater. The presence of MP changed AGS morphology and worsened the settling properties of biomass, causing biomass washout from the reactors. At the highest PE load in wastewater, the biomass concentration in the reactor effluent was over 2.8 times higher than in the control.