I just took a quick look at the characteristic curves for FP-4Plus and HP-5Plus in the Ilford literature; they don't look all that different to me except that HP-5 has a bit more of a shoulder. The curves in the Ilford tech sheets weren't all that detailed, however.

Medium speed films like FP-4 are inherently more contrasty than faster films. The usual initial challenge with these is finding a developing time that tames the overall contrast yet still gives you the local contrast/separation you want. The impression of "muddy" and simultaneously "too contrasty" is a bit of a contradiction in terms, but may relate to differences in curve shape, e.g., mid-tone separation is not what you want, but the highlights are right or even too high. Things like this are characteristics of particular films and can't be changed very easily. A staining developer might tame highlights somewhat in a case like this (not sure if that's really your actual situation) or you can just move on to a film you like better.

It may be interesting for you to make the same image with both films a time or two and really compare how the distribution of tonalities are. For me, it's a valuable exercise. When I'm learning a new film, I load holders back-to-back with an old favorite (usually 320Tri-X) and the new film and make two exposures of the same scene for a while. Sometimes I like the new film, sometimes I don't.

Best,

Doremus