All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
This may have been sweat or water remaining from Istinjaa’ (washing the private parts). In any case, it is not ruled that your ablution is invalidated unless you are very certain, a certainty upon which you can give an oath that you have discharged something which invalidates your ablution. Without this certainty, in principle your ablution continues to be valid.
Allah knows best.