We have studied the magnetic properties of self-organized ErAs aggregates embedded in a GaAs matrix using telegraph noise spectroscopy. The electrical transport through the samples is mesoscopic and exhibits universal conductance fluctuations and two-level fluctuations (TLF's). A statistical analysis of a TLF allows us to measure the extremely small magnetization of an isolated ErAs cluster. Rotating the sample in a magnetic field reveals the sixfold magnetic anisotropy expected for ErAs with a rocksalt structure. At high temperatures the switching is thermally activated, while at low temperatures it is the tunneling of the magnetization which dominates. The magnetic-field dependence of the crossover temperature between the thermally activated and tunneling regimes can be explained with a simple model.