Are they real seashells or fake decorations? Its not good to have any sort of real shells or coral in a freshwater tank unless you understand that it can raise the hardness of the water to a great degree and that is what you intend. In fishkeeping it is nearly always desirable to try to have your tank pH and hardness stay relatively close to that of your tapwater.
Anyway, if they are decorations, you just pull them out of the tank (its nice to have some sort of plastic tray (or the large lid of a plastic storage container, that sort of thing) for use next to you when you are pulling things out or trimming plants etc.)... you pull the decorations out and cart them to a sink, where you scrub on them with an aquarium sponge and/or brushes you have for the purpose. I find that white Fluval replacement sponges have a good scratchy consistency that I like for cleaning both glass and other tank items, so I get some of those and cut them up to a nice hand size. New toothbrushes or other little brushes can be good (its important to note with all these things that you don't want to use old items that may have some chemical left over from some other funtion or to use sponges from a store and not realize they've been made to have added soaps or something.) Its important to realize as a beginner that beautiful tanks don't just happen, they often require some good elbow grease and attention each week during maintenance (hopefully though, your algae problem is temporary!)
The solution for the algae on the gravel is simply to gravel clean it, thereby turning it over randomly and causing many of the surface pieces to be covered over, where the algae will die from lack of light. Obviously you can also just do this with your hand, and probably even better than the gravel cleaning cylinder can.
Note also that water changes are also a component of combating algae and even during fishless cycling this is no exception. Even though we don't like to water change too much during fishless cycling, we can do it occasionally and then just recharge the ammonia and go forward. Water changing helps to cut way down on free algae spores. This is not to call attention away from fewer light hours though as that is the main remedy to apply.
~~waterdrop~~
edit: forgot about the plant nutrition: Even though an average aquarium with plants (no special addition of CO2 or bright lights) will provide enough macro and micro nutrients via the fish waste and incoming tap water, the fishless cycling aquarium is a special case because it is both new and also doesn't have fish! Because of this, some very small doses of fertilizer can be a lifesaver to the plants. In the UK I'd recommend finding a bottle of TPN+ (Tropica Plant Nutrition Plus, the Plus part being important) and a bottle of EasyCarbo (or other liquid carbon) and I'd dose the Easycarbo per instructions, whereas I'd only dose a fourth or half the recommended amount that the TPN+ says (the assumption being that their amounts are more for tanks that are begin "pushed" with extra light and CO2, which yours is not.) If in the USA, then Flourish Excel is the liquid carbon and a set of other Flourish ferts can be used to cover the macros and micros (we can help further if you are US based.)