You could develop this into a quite believable reproduction mechanism.
In complex life forms on earth, egg and sperm are formed in a process called Meiosis that reduces the chromosomes inside a cell. 2 of those haploid cells combined have one complete set of chromosomes and create a baby.
If your humanoids produce haploid cells on the outer layer of their skin, they become part of the slime surrounding them. Combine 2 sets of slime to combine haploid cells (like egg and sperm) and you have a really big mass of diploid (fertilized) cells that could form into an embryo.
There needs to be a catalyst that prompts two cells of different individuals to combine into one cell to rule out self-insamination.
One problem might be how to determine which cells should form which organ. If some cells in the middle decide to become an arm while some cells on the exterior decide to become a heart, the embryo won't be able to survive. It would probably take some time and hormones and whatnot to control the development of the embryo. It would also be best to keep the slime/embryo save in a womb-like structure during this process. The slime needs time to settle or dissipate and let the cells combine into a living organism.
Since slime dissolves into water, your humanoids shouldn't produce haploid cells all the time. That would be a waste of energy and not viable from a evolutionary point of view. It's far more believable for them to have a hormonal cycle or breeding season in which the haploid cells are released into the slime.