Larry, what is mushroom mulch?
It's what they use to grow mushrooms. It is sold to garden centers after it has been used for growing mushrooms. It is still pretty hot and should set for a while before growing in it. And I don't use it for seedlings.
Here is an link about using it to grow tomatoes.
Mushroom Compost & Tomatoes | Home Guides | SF Gate
Or just the important parts.
Mushroom Compost Content
The precise composition of mushroom compost varies from source to source, but in general it is composed of a customized mixture that includes hay, straw, spent brewer's grain, peat moss, chicken manure, corncobs, potash, gypsum, cottonseed meal, soybean meal, lime and ammonium nitrate. Before being used as a growing medium for mushrooms, piles sit for up to a month to "cook," or undergo a chemical process that results in finished compost.
Benefits
Mushroom compost sold in garden centers and agricultural supply stores has already been used to grow mushrooms, but it still has a variety of nutrients and minerals valuable for garden plants and vegetables. In addition to increasing the water-holding capacity of garden soils, it raises the pH (alkalinity) of soil, which can be beneficial for gardeners in areas with particularly acidic soils. Mushroom compost is also rich in calcium, a deficiency of which leads to blossom-end rot on tomato fruits.