Diminishing marginal utility is simply one "kind" of marginal utility. Marginal utility can be increasing, decreasing, or stable.
Marginal utility is simply the added benefit that a consumer gets from consuming the next unit of some good or service. In other words, if I have bought and eaten two sandwiches and I buy and eat another, the extra benefit I get from the third sandwich is my marginal utility from consuming the third sandwich.
Diminishing marginal utility comes if I get less marginal utility from consuming the next unit than I got from consuming the previous one. Using the sandwich example, if I got more marginal utility from the second sandwich than I do from the third (maybe I'm getting too full and didn't really want the whole third sandwich), my marginal utility is declining.
So, there is not really a difference between these two. It is just that diminishing marginal utility is one "kind" of marginal utility.