It is common to need to clean a slipway to remove fouling and algae. However, you should be aware that what you use to clean a slipway is regulated.
Any biocidal products containing active substances with the intention of destroying algae require Health and Safety Executive (HSE) approval. Any such product approved by HSE for use in and around water is accepted by the Environment Agency so long as the product is used in accordance with the conditions on the label. The MMO provide information on this here.
You can check HSE approvals here, although this does not list whether the product has been approved for use in water. Any supplier of a slipway cleaning product should be able to provide you with their certificate from HSE including whether in water use has been approved.
Please be aware that we have no feedback or experience of the products on the HSE approved list, and are merely providing information as to their approval status with HSE. If you have any further information or feedback on using a slipway cleaner, please contact us at firstname.lastname@example.org.