Hydroseeding over existing grass is possible, but it requires careful preparation. The existing grass must be mowed as short as possible and the soil should be loosened by raking or aerating. This allows the hydroseed mixture to make good contact with the soil, which is essential for germination.

It's also important to note that hydroseeding over existing grass may not be as effective as seeding on bare soil. The existing grass can compete with the new seeds for resources, potentially reducing the success rate of the hydroseeding.

If the existing grass is in poor condition or if you want to change the type of grass, it may be better to remove the old grass before hydroseeding.