You can, but you don’t have to. If you like your dal looking clear and smooth, skim the foam during the first few minutes of boiling. If you are cooking dal for an everyday meal and don’t mind the appearance, you can simply let it cook.
The more important steps are washing the dal properly, using adequate water and cooking it thoroughly. Soaking can also help with cooking time and may reduce some water-soluble compounds.
The next time your dal starts producing that familiar white foam, there is no need to treat it like an emergency. It is mostly a visible reminder of what makes pulses such interesting foods: their proteins, starches and naturally occurring plant compounds are interacting with heat and water right inside your pot. In other words, you can skim the foam for a cleaner-looking dal, but you don’t need to remove it because you think it is poisonous or unhealthy.