Some questions on mature tank

[y.roc30]

Just the thread I was looking for. I bought a mature, cycled for several years 38 gallon tank with two 3-4 inch damsels. There is a very deep maybe 4inch substrate made of crushed coral bed and around the base through the glass there is a lot of unsightly brown gunk growing in the front and sides of tank. Now my question is can I dig out underneath the substrate and clean this off the glass without causing a nitrate spike and harming my fish. Currently have a coral beauty, brittle star, and a host of crab and snails. Already lost a algae blenny after several hours of having home from LFS My other question is can I eventually switch to sand substrate without having to remove and re-cycle my tank. Thank you for listening...