Mold or moss?? There is a difference but neither is desirable and you would need to correct the conditions that encouraged either of them to develop before you would have success overseeding.

Jerry where do you live (not your zone)?

What kind of grass do you have/want?

Is there much shade in the area where you want to improve the lawn?

You probably do not need any soil in the topdressing. Soil will change your drainage while the compost will not.

shaded area, buf.n.y. afternoon sun, had some pine trees which caused problems with my plumbing (they are going to be removed). roots all over the lawn. tried to plant shade grass around pines with little success. going to top dress with topsoil-compost mix and reseed. lawn has a nice slope.

With the shade the only grass type that will work is fescue. With the slope you could use soil to topdress but topdressing is not needed at all. I would still roll it down or walk it down to press the topdressing down onto the seed to help with moisture retention. Be sure when you water not to wash the seed and topdressing down the slope. And pray for no rain until the grass comes in.

You should be warned about seeding new grass in the spring. Spring is when crabgrass seed is germinating. I don't think you are far enough north to avoid that problem. When you seed this time of year the crabgrass germinates and comes up quickly giving you the impression that you are a lawn genius. The best time of year to seed northern grasses is in the late summer and very early fall after the crabgrass seed has stopped germinating. Furthermore, new grass plants started in the spring will suffer from the summer heat. If you have a cool summer then no problem, but if you have lots of days in the high 90s, then the fescue will suffer giving the crabgrass a better chance to take over. You'll know by July how it worked. If it ends up being all crabgrass, then take the hit for this year and redo it in late August. That will avoid the crabgrass and give the grass a good 9 or so months to harden up for next summer's heat.

fine fescue would be great if you don't have a lot of traffic. very cold tolerant too. I've seen it grow in quite a bit of shade but not wet. spring seeding might actually work because the shade offers some protection from the heat of summer.