The bacterial concentration will be very high and you do not know which bacteria you are growing. Strictly speaking, you are already in Biohazard category 2 (out of 4). For whatever reason, hay infusions seem to be a common school microscopy project, but I highly disadvise to make one.
OK, let’s start at the basics. The number of ciliates in pond water is often relatively low. A hay infusion is a way to enrich these organisms. Hay infusions are nutrient media to grow ciliates, such as Paramecium, which are interesting microscopy specimens. A little bit of dry grass (i.e. hay) is boiled with some water. The water will turn brownish-green, and look a little like tea. If you now add some pond water containing bacteria and ciliates, the bacteria will start to reproduce and serve as food for the ciliates. The solution will turn turbid over the next couple of days, which is a sign of microorganism growth. If conditions are ideal, the number of ciliates will increase, keeping the number of bacteria low. Hay infusions are difficult to stabilize. The number of bacteria might increase too rapidly, lowering the oxygen content of the infusion and preventing the growth of the ciliates. In any case, the composition of the organisms changes over time.